Riot officers fired pepper spray and non-lethal rounds to repel stone-throwing anarchist youths marching on a summit of the world's most powerful leaders.

Riot officers intervened after a 1000-strong crowd, led by black-clad hardliners wearing goggles, helmets and masks and brandishing anti-capitalist banners, began to trek across Pittsburgh towards the G20 conference venue.

The Group of 20 is a forum for the world's biggest developed and emerging economies and its meetings are a magnet for anti-capitalists opposed to what they see as an undemocratic body promoting inhumane free market policies.

Police in riot gear blocked the main roads leading to downtown Pittsburgh from the gritty neighborhood from which the demonstration set off, forcing groups to divert onto side streets as they sought a gap in the cordon.

About half-an-hour into the march, police began broadcasting a pre-recorded announcement in English and Spanish, declaring the protest was an "unlawful assembly" and ordering the crowd to disperse.

Riot police warn strikers

"If you do not disperse you may be subject to arrest or other police action. Other police action may include actual physical removal, the use of riot control agents and or less-lethal munitions which could cause injuries."

Some heeded the police warning, but others peeled off and ran up Pittsburgh's maze of streets, looking for alternative routes to the summit. Points: G20 agenda.

Riot police blocked a group of several hundred on a narrow side street and fired pepper gas grenades at them after half a dozen youths raced down the road, pushing a rubbish dumpster (wheeled skip) towards the line of officers.

The bulk of the march melted away, coughing, sneezing and with tears streaming from their eyes, but a few pockets of diehards ran scattered through the working class district of Lawrenceville, taunting police lines.

"They pushed us into a side street in a residential area and then shot tear gas at us. They shot like three cannisters," demonstrator Ross McCoy told AFP. Police later said the substance used was pepper spray.

As the now-scattered demonstration ground on into the late afternoon, police again opened fire on a small crowd in the northern part of Pittsburgh, using what the security forces said were non-lethal "bean bag rounds".

"In response to having sticks, bricks and rocks thrown at them in the Shady Side neighborhood of Pittsburgh, police responded with bean bag rounds and dispersed the crowd," FBI agent Bill Crowley told AFP.

Fifteen people were arrested during the protests, police said, although they were unable to give details.

"We're still processing them"

"We're still processing them," Ruth Miller, a spokesperson for the Joint Information Center for law enforcement, set up for the G20 summit, told AFP.

The protesters said they were tracking 23 arrests and called for a vigil Thursday evening at the county jail in downtown Pittsburgh.

While the anarchists clearly drew inspiration from previous protests at global summits, they failed to trigger such large-scale disturbances.

"It's very badly organised by a bunch of young kids," said Pittsburgh resident and demonstrator Fred Marshall.

"It was inevitable that we were going to be dispersed. Their ideas are good, but their voices weren't heard because they were so loosely organized.

"This isn't Europe, we don't do things like they do there," he said, citing the G20 riots that rocked London six months ago.

Lifelong Pittsburgh resident Eileen Leist watched from her front step as the demonstrators walked down Liberty Avenue towards around 50 riot police.

Do not harm them

"Let their message be heard, but don't harm anyone or anything," Leist said.

The main message the mass march wanted to convey to the world leaders at the G20 was that human rights and dignity were more important than capitalism and profit margins, several of the demonstrators told AFP.

"The people who developed the system that's falling apart and trying to fix it? That's just crazy," said Sondra Perry, an art student from New York state.

"Our message here is about climate change, poverty, capitalism: they're all very intertwined and it's time that we all understand that if we are going to do anything, we have to work together," she said.

Paul Erb, a student from Ohio, said he had come to Pittsburgh to protest against a system that puts monetary profit before all else.

"I'm here because the current global economic system, which they are meeting about, values profit over people," Erb told AFP.

"Profit, power and control have become more important than human lives. But that's not what we were raised to think is right. That's not where we come from," the 24-year-old said.

The city had put in place a massive security operation, with police drafted in from across the country, and National Guard troops in camouflage uniforms manning a ring of concrete road blocks around the meeting site.

"We're here to protect everyone, including the civil protesters. Those who want to use violence spoil it for everyone," said Tim Huschak, a bicycle-riding police officer drafted into Pittsburgh from the nearby town of Clairton.

Scattered protests continued in Lawrenceville and around the University of Pittsburgh as night fell, but the city seemed to have been spared the summit violence that hit London six months ago, Genoa in 2001 and Seattle in 1999.