By CHRISTOPHER MARQUIS

GUATEMALA CITY - The gas station attendant was horrified. In front of him, a man, shot through the jaw and chest, had collapsed in a bloody heap over the steering wheel. The attendant read with astonishment the hurriedly scribbled note:

"Please help me," the note begged. "I'm an adviser to Cerezo."

With that desperate plea, Luis Posada Carriles - Bay of Pigs veteran, fugitive from a Venezuelan prison, former aide in Oliver North's contra supply network resurfaced briefly in February from the spy-filled netherworld where he has lived most of his adult life.

Just as quickly, he disappeared again, vanishing in March from the El Pilar clinic where he was last seen.

Behind, he left another adventure to be told, filled with claims of clandestine activities, political intrigue and, perhaps, assassination.

Two days later, Guatemala's President Vinicio Cerezo denied any links to Posada, but there is no doubt, say diplomats and government officials, that Posada was the president's man.

His mission was so secret that it even surprised government security agents investigating the attack.

Posada is one of Cuban Miami's more fascinating warriors. Born in the port city of Cienfuegos, he has slipped m and out of some of the most notorious actions in the long, fruitless exile war against Fidel Castro, often hiding his true identity. For his work in Guatemala, Posada used the name Juan Jose Rivas Lopez.

He was Ramon Medina when he helped refuel contra supply aircraft at El Salvador's Ilopango airfield, disappearing when the Iran-contra scandal began to unfold.

It was under his own name that he was imprisoned to Venezuela in 1976, along with Orlando Bosch, for the in-flight bombing of a Cuban airliner. Unlike Bosch, Posada has never been acquitted in the case; he is out of prison because he escaped in August 1985.

Government officials and foreign diplomats in Guatemala say that since his role in the contra supply network was revealed four years ago, Posada has undergone plastic surgery to alter his facial features. He has shaved off a familiar beard, gained weight and has curlier hair than before.

He carries four passports, testaments to his contacts over the years: one Venezuelan, one Panamanian, one Salvadoran and one American.

In Guatemala, in the February incident, there was no doubt of Posada's importance. Two Cerezo confidants, Francisco Ramirez, head of the Guatemalan telephone company, and Rolando Castro, the director of Guatemalan customs, raced to the gas station to oversee Posada's care. Presidential guards and National Police guarded his hospital room night and day.

Diplomats say Posada was part of a team of espionage experts working exclusively for Cerezo, independent of the Interior Ministry and the National Police.

The special squad, counting perhaps a dozen men, was financed by donations from West Germany and a discretionary fund handled by Cerezo. It boasted the most sophisticated surveillance equipment in the government, the sources said.

Posada's team "could have spied on everybody - embassies, suspicious countries, political parties," said one diplomat briefed by Guatemalan intelligence. "But its first target, without a doubt, would be the armed forces and the top military chiefs."

Cerezo, who has survived several military rebellions, has steadily voiced fears he will be ousted by a coup d'etat.

Gen. Hector Gramajo, Guatemala's defense minister and the nation's top military officer, confirmed that Cerezo insiders had tried to establish a parallel security force. Gramajo said the effort was spearheaded by former Interior Minister Juan Jose Rodil. But it quickly crumbled, Gramajo said, when Rodil and his associates began using the new force for personal gain and even kidnapped the daughter of a banking executive to raise cash.

"They were thugs. They became bandits," Gramajo said. "They'd seen too many movies, and they got off track."

Gramajo declined to comment on Posada. Rodil, who was traveling in Washington, did not return messages left at his office.

In Guatemala's darkening climate of political murder and human rights crimes, foreign diplomats are meeting to share concerns and speculate about the possible role of a new invisible force answering to a vulnerable president.

"You can arrive at some frightening conclusions," said one Latin American diplomat.

Whatever the truth about the squad, all the rumors and allegations, the adventure, the brush with death, was vintage Posada.

For Latin American intrigue, few characters would be more aptly cast. Posada carries with him the image of a rogue spy, a man of limitless resources. Along with Bosch, he is widely seen as an anti-Castro crusader who never gave up.

But, whereas Bosch made his battles public - piling up weapons convictions along the way - Posada became a behind-the-scenes manipulator and technician par excellence.

"Their styles are different. Luis has moved at the level of governments. He's more militant" than Bosch, said Emilio Caballero, a childhood friend.

Bosch, who illegally flew to Miami after his release from a Venezuelan prison, has been held for more than two years by U.S. immigration officials. Posada, fleeing from his Venezuelan cell, has remained active in the anti-Castro cause, a phantomlike figure in the thick of the fight.

Posada's militancy dates to the 1961 Bay of Pigs invasion, in which he took part as a demolitions expert. After that, he underwent basic training in the U.S. Army at Fort Benning, Ga.

His first, and most enduring, relationship as a spy began in 1965, when he joined the CIA, according to a directory of the agency published in 1977.

Posada moved to Venezuela in 1967 and joined that country's intelligence and secret police agency, known as DISIP. He remained on the CIA payroll and informed the CIA of DISIP activities.

He also provided the CIA with information on Cuban exile groups intent on overthrowing Castro, according to the CIA document. It would not be the last time his credentials as a hard-liner permitted him to infiltrate anti-Communist organizations.

Respected for his professionalism, Posada rose quickly to become a sub-director of DISIP. But, according to a widely circulated story, Posada's talent was also the cause of his demise: Engaged in domestic spying, DISIP purportedly taped a racy conversation between an opposition politician and his girlfriend.

When the politician, Carlos Andres Perez, was elected president in 1974, "Posada was not, to say the least, one of his favorite people," according to fellow exile and CIA agent Felix Rodriguez.

Posada was fired from DISIP. He then formed his own security company.

In June 1976, Posada arrived in the Dominican Republic for an anti-Castro "summit." Bosch and other militants were present to lay plans and assign attacks. A string of bombings soon erupted in Colombia, Panama and in some Caribbean islands.

Four months later, Posada and Bosch were arrested as "intellectual authors" of the bombing of a Cubana de Aviacion DC-8 plane that crashed shortly after takeoff from Bridgetown, Barbados. Seventy-three people, including Cuba's national junior fencing team, died.

Two Posada associates, Hernan Ricardo and Freddy Lugo, were convicted of placing the bomb. But the cases of both Posada and Bosch languished in Venezuelan courts for a decade, while military and civilian authorities wrangled over jurisdiction.

The highly sensitive case spoiled Venezuela's relations with Castro, who demanded the men be extradited. It also stirred indignation among Cuban exiles, who noted that both men were long deprived of liberty without being found guilty.

Bosch eventually waited out the charges; Posada found a quicker remedy.

His first escape attempt probably came in 1982, when he and Ricardo managed to flee the San Carlos military prison in Caracas. They sought asylum at the Chilean Embassy, hoping that Gen. Augusto Pinochet would appreciate their anti-Castro struggle. But they judged wrong; Pinochet's government handed them back to Venezuela.

The government promptly split up the suspected conspirators, sending Posada to the San Juan de los Morros jail, about 60 miles south of Caracas. On Aug. 18, 1985, the remote prison became the site of a spectacular jailbreak.

Posada escaped "in Olympic fashion," one Venezuelan official recalls. "Dressed in a neatly pressed suit and tie, with suitcase in hand, he walked through the front door." A plane waiting nearby whisked him to parts unknown, the official said.

Venezuelan authorities later said that Posada had systematically bribed key prison officials, paying some $28,600 for his freedom. Friends in Miami, including fellow Bay of Pigs and U.S. Army veterans, took up collections.

With the help of a Venezuelan friend, Posada traveled to the Netherland Antilles using forged documents identifying him as Ramon Medina, according to Rodriguez, who first met Posada in basic training at Fort Benning.

From there, Posada headed for Central America, where he joined Rodriguez in an operation that would provoke the worst crisis of the Reagan administration.

Using the Ilopango Air Base in El Salvador as a springboard, Rodriguez supervised the covert resupply of contra rebels battling the Sandinista government in Nicaragua. The U.S. Congress had barred U.S. officials from providing such aid at the time, but the program was overseen by Oliver North from his office in the Old Executive Office Building.

Rodriguez says he assigned Posada to be "day-to-day manager" of the operation.

"He ran the four houses (later cut back to three) we used for the American flight crews," Rodriguez wrote in his memoirs. "He saw to it that they were stocked with food; he hired maids and drivers to get the crews back and forth from Ilopango Air Base. Occasionally he even accompanied them on their runs when someone who spoke Spanish was required."

Eugene Hasenfus, the American resupply pilot who was captured after he was shot down by Sandinista gunners on Oct. 5, 1986, identified both Rodriguez and Posada as U.S. agents.

Posada dropped his Medina alias and apparently went to work for then-President Jose Napoleon Duarte of El Salvador, under the Rivas Lopez name. His connection with Duarte was secret. Cabinet aide Gerardo LeChevallier said he knew of no one matching Posada's description who advised the late president.

Posada infiltrated the Salvadoran right and reported back to Duarte on human rights violators and extremists determined to thwart the president's centrist program, according to diplomats and government sources.

It is unclear whether Posada continued working in this capacity when Alfredo Cristiani, a member of the rightist ARENA party, took power last June.

Posada moved to Guatemala last September, after commuting several months to negotiate his role in Cerezo's force.

He moved into an elegant, high-rise apartment in the Vista Hermosa neighborhood. The housing was paid for by the presidency, which had received several million dollars from West German donors interested in protecting Cerezo's Christian Democratic government, according to a Latin American diplomat.

Hired as an expert in electronic surveillance, Posada received his salary through the state telephone company, GUATEL, which became his base, sources said.

On Feb. 26, Posada was driving to work at 9 a.m. in a black Suzuki jeep when two cars pulled alongside on the tree-lined Vista Hermosa boulevard. Gunmen fired at least 40 nine-millimeter bullets at Posada's car, then sped off.

The assault, witnesses recalled, occurred in eerie quiet; the automatic weapons had silencers. Students at a nearby university puzzled at the sight of cracking tree branches overhead. "A very sophisticated attack," said one government official.

Posada, bleeding profusely from three wounds - in the hip, the left side of his chest and the jaw - managed to drive half a mile to an Esso gas station.

Station owner Mario Chavez was astonished. "He made the sign of the cross," Chavez recalled. "Imagine it. He thought he was going to die."

Posada gestured frantically for a pencil and paper. "Allergic to penicillin," he scribbled. "I'm an adviser to Cerezo." He wrote down a phone number. A woman answered when Chavez called.

Within 15 minutes, the head of GUATEL and the director of Guatemalan customs appeared with an unidentified woman to watch over Posada, government sources said. Paramedics took him to El Pilar sanatorium, an expensive facility several blocks away, where Posada would remain for a month, with presidential guards and National. Police outside his door, employees said.

In Miami, Posada's wife, Nieves, heard reports that Posada was dead. She telephoned Caballero, Posada's childhood friend and her attorney. "She called saying, 'Havana radio says Luis was killed,' " Caballero said. "She was terribly frightened."

Nieves Posada left Miami to be at her husband's side, Caballero said. Efforts to contact her were unsuccessful.

From his hospital bed, Posada tried to guess the identity of his would-be assassins. They were hit men tied to rightist extremists in El Salvador, he told investigators. Or else they were Castro agents. Or they might have been Nicaraguan Sandinistas doing Castro's bidding, he said, according to a diplomat briefed by officials.

The man of many crusades then vanished. Without him, police might never determine which of his many enemies tried to kill him.

"If Posada were killed in 1986 [the year after he escaped from prison], I'd have said it was the Cubans," said a Western diplomat who has followed Posada's career. "But if he gets killed in 1989 or '90, I'd say there's a lot of people, who might want to take him out."