By LYDIA POLGREEN
 
PORT-AU-PRINCE, Haiti, Feb. 12 — Militant supporters of President Jean-Bertrand Aristide blocked a demonstration planned by civic opposition groups in the capital on Thursday, erecting barricades of flaming tires and throwing rocks at anyone who tried to breach their blockade.

Militants loyal to Mr. Aristide and his Lavalas party started gathering in the square late Wednesday night, and Thursday morning hundreds of them threw rocks, taunted and shouted at opposition protesters.

"With everything I have got I will fight them," said Willy Dumeria, 30, an Aristide loyalist who spent the night in the square where the opposition protesters were to gather.

As he spoke he pulled a sharpened steel rod from his trousers and brandished it. In his other hand he held a picture of Mr. Aristide.

"They don't respect the government," he said of the protesters. "But we will take care of them and save our power. The opposition, they are terrorists."

Opposition groups said they saw the planned march as a crucial test of Mr. Aristide's intentions as the country convulses with armed uprisings in towns along the western Caribbean coast.

Mr. Aristide has said that those who oppose his government are free to demonstrate, but the police and pro-Aristide gangs have often blocked marches, firing tear gas and sometimes bullets into the crowds. The protest would have been the first since uprisings began sweeping the country a week ago, killing dozens of people and bringing Haiti to the brink of chaos.

Residents here said they heard gunfire in the capital, Port-au-Prince, early in the morning. Later, tensions were evident among people in the square.

Opposition members who showed up to march said they were beaten and robbed by furious mobs.

"I just wanted to be a member of the demonstration today because I am afraid of the bad things Aristide is doing," said one of the thwarted protesters, Emmanuel Jean François, 27, a university student.

He said that Aristide supporters had surrounded him but that he had managed to flee their blockade. "They took my wallet, they took my phone, they were going to kill me," he said. "We students have risen up because they tried to kill us every day. We have no freedom."

At a police station overlooking the square where the mob gathered, officers watched but did not interfere. Blockades put up to keep protesters from reaching the square smoldered, snarling traffic.

At a news conference later in the day, opposition leaders denounced Mr. Aristide and called on the United States and other nations to do the same, saying he had not lived up to his pledge to allow dissenters to march.

"We decided we needed to take to the streets to show we offer a nonviolent option," said Andy Apaid, a businessman who represents the Group 184, a prominent opposition group. "But armed thugs invaded where we were supposed to gather, under the nose of the police station.

Opposition leaders said the march was a chance to demonstrate their groups' strength and emphasize their nonviolent approach in the capital. They have struggled to distance themselves from the violent uprisings sweeping through cities and towns along the northern coast.

Mr. Aristide has labeled opposition groups "terrorists," and he has said that civic groups that espouse nonviolence are secretly supporting the armed militants.

Mr. Apaid said opposition groups planned to hold a march on Sunday, beginning at a church in Pétionville, a suburb perched high atop a hill above the capital.

In Washington, Secretary of State Colin L. Powell said Thursday that the administration was discussing the possibility of asking Canadian or Caribbean police forces to go to Haiti to help establish order.

He denied that the Bush administration was seeking to replace President Aristide. "The policy of the administration is not regime change," Mr. Powell said.