By LYDIA POLGREEN and TIM WEINER
 
PORT-AU-PRINCE, Haiti, March 8 — Interim President Boniface Alexandre was installed in the National Palace in a brief, awkward ceremony on Monday, as hundreds of supporters of Haiti's exiled president, Jean-Bertrand Aristide, marched through the capital, chanting "Aristide or death!"

A power struggle continued in the city's streets and its salons with the search for a new prime minister, a day after Aristide loyalists opened fire on anti-Aristide demonstrators near the presidential palace. Six people were killed and about 30 wounded. United States marines said on Monday that their return fire had killed one gunman.

The United States ambassador, James B. Foley, called the deaths of opposition marchers and a foreign journalist tragic, but praised the actions of the marines and French troops, saying the city's security situation was rapidly improving.

"It is really a matter of time before the multinational interim force is able to exercise authority over the entire city," Mr. Foley said in an interview on Monday with The New York Times. "Each day we are stronger and each day we are in a better position to provide security."

In his first public appearance since Mr. Aristide fled Feb. 29, Mr. Alexandre called on Haitians to unite for the good of the nation.

"We are all brothers and sisters," Mr. Alexandre said, his voice unsteady, as he read from a prepared text, his eyes never leaving the paper. "We are all in the same boat. If it sinks, we all sink together."

Mr. Alexandre, as Haiti's chief justice, became interim president under a constitutional provision for the top judge to fill any vacancy in the presidency, with Parliament's approval. But the legislature was disbanded in January, so he has not been approved.

The United States is hoping for a stable, pro-American provisional government to emerge from eight days of struggle following President Aristide's departure under American pressure and on an American aircraft.

A seven-member council set up to restore civil authority under a plan by the 15-nation Caribbean Community interviewed three candidates for prime minister on Monday.

The three are Hérard Abraham, a lieutenant general in the now disbanded Haitian Army, who briefly led an army junta in 1990 but ceded power to a civilian; Smarck Michel, a former Aristide prime minister; and Gérard Latortue, an international business consultant who served as foreign minister in 1988.

Prime Minister Yvon Neptune, the last holdout from Mr. Aristide's government, has been taking shelter in his office since rebels destroyed his home, according to Yvon Feuille, who was until January was president of the Senate. Mr. Feuille said he expected a new prime minister to be appointed as early as Tuesday.

The violence on Sunday led to the first armed action by international troops since they began arriving in strength on Feb. 29, the day Mr. Aristide, Haiti's first democratically elected president, went into exile.

Col. Charles Gurganus, a spokesman for the marines stationed here, said the Americans opened fire on an armed man firing at them and at demonstrators, as well as on another gunman who escaped.

He defended the marines, who were criticized by many Haitians for moving too slowly to quell the violence. "Right now this not a peacekeeping mission," Colonel Gurganus said. "It is our job to try to establish the stability which will allow the transition to the peacekeeping mission."

An anti-Aristide rebel leader, Guy Philippe, said he might rearm his men in the wake of the violence on Sunday, raising the specter of street warfare between Mr. Philippe's army and armed Aristide supporters, known as chimères.

"If they don't disarm the chimères, I will reassemble my men and take up arms again," Mr. Philippe said in an interview with Radio Vision 2000.

The weapons saturating the country pose a daunting problem for American military officers and the new chief of the Haitian police. A senior Western diplomat said "the Aristide government was madly distributing weapons into the hands of lawless elements" as the rebels seized half the country last month. "We have a tremendous security challenge on our hands."

On Monday, the capital's streets were tense even as marines and French troops patrolled. Less than a mile from the airport, looters sacked the city's industrial park, carting off whatever they could carry.

Downtown, near the city's cathedral, about 75 marines warily patrolled neighborhoods loyal to Mr. Aristide. They were taunted by residents, and after a brief confrontation, they left the neighborhood.

The country's new police chief, Léon Charles, a former Haitian Coast Guard commander, said in an interview on Monday that disarming militants on both sides of the conflict is one of his top priorities.

Mr. Charles must also rebuild a force corrupted by Colombian drug traffickers, who pay millions of dollars a year to Haitian officials to ship cocaine through Haiti to the United States, the State Department says.

The United States government estimates that the value of bribes paid in Haiti by cocaine kingpins is nearly three times the government's spending on police, courts and justice, and that nearly a quarter of the Colombian cocaine reaching the United States flows through Haiti — almost 80 tons a year.

"It's not a recent phenomenon," Paul E. Simons, a senior State Department official, said in Washington in September.

"Haiti is acting as something of a magnet in the Caribbean for cocaine flows," because drugs flow most easily through failed or weak states, he said.

Two longstanding pillars of American foreign policy here have been preventing the flow of refugees and Colombian cocaine from Haiti to the United States, State Department officials have repeatedly said.

Haiti has seen 15 governments and army juntas in the past 18 years. Throughout those years, even under Mr. Aristide, who was democratically elected, "a small economic elite has supported a predatory state" shored up by "pervasive repression through army, police and paramilitary groups," the World Bank said in a 2002 study.

In his 15-minute address at the National Palace, Mr. Alexandre urged all combatants to overcome Haiti's bitter history and put down their weapons. "Remember that no one has a right to do justice for himself," he said.