Protesters smashed shop windows and threw rocks at police on Thursday as officers in riot gear used pepper gas and batons to disperse their march against capitalism at the Group of 20 summit in Pittsburgh.

Protesters wore bandannas and goggles and held aloft a large black sign declaring "No hope in capitalism" and another saying "Kick Capitalism While It Is Down."

One sign simply said "I'm mad as hell."

Protesters at Group of 20 summit in PittsburghProtests - usually against some aspect of capitalism - have often marked summits since trade talks in Seattle in 1999, when demonstrators ransacked the centre of the city, targeting businesses seen as symbols of U.S. corporate power.

"We have seen police use rubber bullets, batons and gas," said Noah Williams, a spokesman for the anti-capitalist Pittsburgh G20 Resistance Project.

During the evening rush hour in Bloomfield, Pittsburgh's Little Italy, about 300 rowdy protesters remained from a crowd that originally numbered 2,000.

The crowd broke windows at Boston Market and KFC fast-food restaurants, a BMW dealership and a Fidelity Bank in the area, about a mile from the fenced-off convention centre where the G20 talks were taking place.

Police in body armour with plastic shields threw pepper gas canisters and fired pellet-filled "beanbags" to disperse the protesters, charging in to make some arrests.

By nightfall, only a few demonstrators were left wandering downtown streets but a series of actions were planned for Friday morning at locations including Starbucks, Gap, McDonald's and banks.

Leaders of 19 leading developed and developing economies and the European Union were meeting on Thursday and Friday in Pittsburgh to discuss how to improve financial reforms to avoid another global economic crisis.

A G20 meeting in London in April drew thousands of people to a protest that began peacefully before turning violent, with one person collapsing and dying.

‘Non-lethal force'

Four hours of clashes in Pittsburgh began when police dispersed the 2,000 people who had gathered at lunchtime for a march.

"You must leave the immediate vicinity regardless of your purpose," police said over bullhorns throughout the march and warned that gas and other "non-lethal force" would be used.

With protesters sent down various streets by police, the two sides soon clashed in the Lawrenceville neighbourhood.

Protesters threw bottles and rocks and police responded by sending five to 10 canisters into the crowd. The sharp smell of the gas irritated the eyes and throats of protesters, some of them vomiting as they ran.

Protesters at Group of 20 summit in PittsburghUS Secret Service spokesman Darrin Blackford said the gas used on Thursday was OC Vapor, which contains the active ingredient in pepper spray and causes the eyes to tear.

He confirmed three people were arrested and said he had unconfirmed reports of another four. He denied local media reports that police used rubber bullets on the crowd.

The marchers overturned dumpsters and hurled anything they could find at police, who gave chase and broke them into ever smaller groups.

Ear-splitting loud sirens sent protesters fleeing.

"We're here to put pressure on the G20 to ultimately abolish global capitalism," said a 24-year-old man from Delaware, who declined to give his name.

Justin Hershkovitz, 26, a student from Michigan, complained about the police tactics as he ran from the officers.

"This kind of force has been used as an option of first resort by cops (at summits) in Italy, London and now Pittsburgh," he said.

As protesters ran through Pittsburgh's Strip District, some residents stood in their doorways cheering. Others jeered.

"Get a job," one man shouted at a running demonstrator.

- Reuters