Police fired rubber bullets, water cannons and tear gas at hundreds of rock-throwing activists Saturday while most of the nearly 30,000 demonstrators marched peacefully through this picturesque city protesting a proposed free-trade pact.

In a second day of running clashes with police, protesters shook the chain-link and concrete wall encircling a 34-nation summit and pelted authorities with stones and sand-filled bottles. Officers charged with night sticks and opened fire with rubber bullets, tear gas and water cannons, blowing one man back on the pavement.

Police said nearly 30,000 protesters crowded around the summit site, with 2,000 growing violent. In two days of unrest, at least 34 police officers were injured, as were 57 demonstrators. There were at least 150 arrests, police said.

Protesters tied a long rope to one section of the fence in an attempt to tear it down. At another point they used wire cutters and tore the barrier down with their bare hands, but a graveyard fence still stood in their way. Riot police took up positions among the tombstones to defend the perimeter.

Another group tried to breach the fence a few blocks away but also was held back with water cannons and tear gas.

The violence came as two peaceful marches wound through the city. Thousands of people converged on Quebec from across the hemisphere and Europe to protest the Summit of the Americas, where President Bush  and other leaders debated a zone known as the Free Trade Area of the Americas.

A delegation of activists trying to breach the gate met up with one of the marches and urged protesters to join them at the clashes. But the marchers, who filed through the 17th-century city chanting ``solidarity,'' said their march was peaceful and continued on their planned route.

At a separate march across town, a carnival atmosphere accompanied a crowd that included people of all ages. Cheerleaders with pompoms led anti-free trade chants and a float carried a guillotine draped with the American flag.

One woman painted her bare chest with an anti-trade slogan.

At one spot of frequent clashes between police and demonstrators, police brought in snarling dogs and positioned them just inside the fence in case any demonstrators broke through.

Authorities also positioned snowmaking machines next to water cannons. The snow machines can hurl water farther than the cannons.

Protesters broke open the fence in one area nearby, but police quickly rushed in and stood shoulder-to-shoulder on both sides of the break until the fence was repaired.

Police continued to fire and hurl tear gas canisters into the crowds. But demonstrators often picked up the canisters and threw them back at the police. Several demonstrators used hockey sticks to slam the canisters away from them.

At one point, a group of black-clad activists began to throw wooden barricades through the windows of a bank, shattering them. Other protesters quickly surrounded them and booed.

``Go and confront the police. Don't destroy property. It gives us all a bad image,'' admonished Sel Burrows, a 57-year-old retiree from Thompson, Canada.

He turned to a journalist. ``They're just crazies,'' he said. ``They don't represent the rest of us.''

Organizers of that march asked police to stop firing tear gas as they passed near the flashpoints, saying children in the protest group could be sickened by the gas. Police did not appear to let up in their barrage.

The protesters represent a diverse range of activists - organized labor, human rights organizations, environmental groups and others who say the trade talks should be held in public instead of in a locked conference center. Many said that forced them to express their opposition through street protests.

There also were anti-summit demonstrations Saturday in the United States. About 3,000 people shut down a stretch of the U.S.-Canadian border for more than four hours in Blaine, Wash. In San Diego, about 1,000 people demonstrated at the U.S.-Mexican border. Other protests occurred in Vermont and at the entrance to a tunnel that links Detroit and Windsor, Ontario.