Black-clad anarchists protesting the G20 summit in Toronto smashed storefronts and wrecked a police car on Saturday, while riot police donned gas masks in anticipation of an escalation of violence.

Masked protesters smashed windows of a bank branch and other buildings, including a Starbucks coffee shop, and set a police car ablaze. Two media trucks were also damaged.

An Emergency Services spokeswoman said at least three people had been wounded in the protest, but that paramedics were unable to reach them due to the protest.

By midafternoon, demonstrators numbered in the thousands, while hundreds of police massed and authorities shut down public transit and blocked streets leading into the downtown core of the city.

Most protesters remained peaceful, with many bearing signs and chanting slogans aimed both at the G20 and police tactics.

'Whose streets, our streets,' roared the crowd, while TV images focused on 'anarchists' who had promised to infiltrate the crowd and face off against police at the 10-foot (3-meter) high barrier that encircles the Group of 20 meeting site.

Soon after the demonstrators arrived near the G20 barriers, groups of black-clad protesters appeared to separate themselves from the larger group and confronted the hundreds of police shadowing the march.

TV reports said protesters at the front of the march were hurling objects such as golf balls at police.

A Reuters reporter earlier witnessed some demonstrators chipping pieces off concrete planters lining the path of the march before scuffles between the two sides. Police used plastic shields to shove demonstrators back into the crowds.

Anti-G20 groups have been demonstrating in Toronto all week before the summit of rich and emerging economies, which follows a smaller meeting of Group of Eight industrial nations in the Ontario resort town of Huntsville.

Canada has budgeted more than C$1 billion ($970 million) for security for the two summits.

Police earlier this week arrested a man and a woman a few blocks from the summit site and said they had found incendiary devices.

Overnight, police conducted raids on houses of suspected protest organizers in Toronto and arrested four people on charges of conspiracy to commit mischief, in what police said was a protest-related move.-Reuters