MEXICO CITY, Oct. 8 - Two Cuban diplomats were arrested last month by the Mexican authorities and held incommunicado for a week after they were apparently lured to a secret meeting with two Cuban exiles here, according to Mexican and diplomatic sources.

The Cuban diplomats were said to have been beaten and threatened while in custody.

It is the first incident of its kind known to have involved the two countries, and diplomats here say it has caused deep concern in Havana. Despite their conflicting ideologies, the two nations have long kept close ties and Mexico has always actively opposed Washington's attempts to isolate Cuban in the region.

The two diplomats were informally accused of preparing to deliver explosives to the exiles when they were detained by Mexican security agents in a bus station in this city on Sept. 1. A week later, the two men were given to Cuban officials here and flown to Havana, the sources said.

The two exiles, anti-Castro Cubans, who, the sources said were United States citizens, were reportedly detained at the same time and expelled to the United States. [Duney Perez Alamo and Raul Varandela Estevez].

The sources said that the Cuban Embassy here had denied the existence of explosives and said the two officials were entrapped by political groups interested in disturbing warm Cuban-Mexican relations.

The reported incident has raised questions here about the two countries' future relations, including whether Mexico has entered a phase of aggressiveness toward Cuba or whether Cuba has lowered its guard and become less meticulous in its normally cautious activities in Mexico.

To date, officials on all sides who are parties to the incident appear anxious to maintain secrecy. Only a passing reference to the "disappearance" of two Cuban officials was made in Mexican newspapers. A Cuban Embassy spokesman here referred all questions to the Mexican authorities.

A spokesman for Mexico's Interior Ministry, whose federal security agents made the arrests, denied any knowledge of the case. Similarly, a senior official at the Foreign Ministry said he knew nothing of the event. At the United States Embassy here, a spokesman had "no comment," although sources close to the embassy said embassy officials had been informed of the deportation of the two United Stated citizens.

But the details that have become known in Mexican diplomatic and political circles offer some insight into intelligence operations in this capital, whish has long been known as a center for Central American and Caribbean political intrigue.

The secret meeting between the two Cuban officials, José Ramón Pérez Ayala and Arturo Guzmán Nolasco and the two exiles, whose names could not be obtained, was apparently arranged some time before Sept. 1.

Mr. Pérez, 30 years old, had been in Mexico City for two years, accredited as a diplomat to the Cuban Embassy. Mr. Guzmán, 27, who is believed to he a Cuban security agent, arrived from Havana carrying a diplomatic passport, while the two Cuban exiles traveled from Miami for the meeting, the sources said.

The exiles, whom the sources described as members of an anti-Castro terrorist organization [Cuba Independiente y Democratica (CID)], had offered to sell information to the Cuban Government, the sources said, and the time and the place of the meeting was apparently confirmed once the four men were in Mexico.

The two Cuban officials, the sources went on, took elaborate security precautions, traveling first by embassy car to a place close to Xochimilco, a southern suburb, then by taxi and finally via different subway trains to the rendevous at the suburban Terminal del Norte bus station.

At the moment the Mexican officials met the exiles, all four men were arrested by agents of the Federal Security Bureau who were also waiting, with or without the exiles' knowledge.

The two Cuban-Americans, the sources said, were deported to the United States "within 24 hours" but the Cuban officials, who have diplomatic immunity, were jailed, threatened and beaten while under interrogation by Mexican federal agents.

Only on Sept. 4, the sources continued, was Cuba's Ambassador to Mexico, Dr. Fernando López Muíño, called in by Mexico's Foreign Minister, Bernardo Sepúlveda Amor, and informed of the detention of the diplomats on charges of possessing a bomb which was to be delivered to the anti-Castro exiles. The diplomats were also said to be carrying a small revolver each.

A Mexican military report, the sources said, had described the Cubans' device as a "highly refined explosive," consisting of explosive disks and phosphorus.

According to the sources, the Cuban Embassy said that the "bomb" was nothing more than a small handbag with a built-in device for destroying its contents. Such a devise, the sources quoted Cuban Embassy officials as saying, is commonly used by couriers carrying secret documents.

On Sept. 7 the two detained officials were taken from the jail where they were being held, which was not identified, to a Mexican Government hangar, and left on a Cuban plane.

The accounts leave a number of key questions unanswered. Mexican sources said that some senior government officials here said they believed the incident was a provocation arranged by the United States Central Intelligence Agency to disrupt a planned trip of a Mexican mission to Cuba.

On Sept. 5 in Havana, Mexico's Ministers of Finance and Foreign Trade, Jesús Silva Herzog and Hector Hernández, signed a much-publicized agreement to extend $55 million in Mexican credit to Cuba. The sources said President Miguel de la Madrid Hurtado, after learning on the Sept. 1 arrest, decided that the mission should go to Havana anyway.

But foreign diplomats here said that even if Mexican agents were merely reacting to a tipoff and cooperating unwittingly, this does not explain why senior Government officials let the two Cuban-Americans go while holding the Cuban officials for one week, or why the Cuban officials were interrogated and beaten.

Sources close the Cuban Embassy here said that Cuban officials were less concerned by the possibility of an entrapment prepared by the C.I.A., which it would regard as more normal business, but that they were worried about Mexico's handling of the diplomats and Mexico's insistence that they had a bomb.

In the view of some foreign diplomats here, the current Mexican Administration, which is less supportive of, and more neutral toward, the Central American left than was its predecessor, may have been sending a message to Havana.

Mexico has long offered international support for Cuba, while Cuba does not interfere in Mexican domestic politics. By most accounts, Cuba, which has one of its largest embassies in Mexico, has closely adhered to this arrangement. Mexico has apparently not regarded Cuba's intelligence-gathering and other activities in Mexico as harmful to its own interests.