Police officers stand guard outside the London Stock Exchange as a businessman walks to work Thursday ahead of the G20 summit. (Tim Hales/Associated Press)

Police in London remained on high alert Thursday after arresting more than 100 people connected to widespread demonstrations against the G20 economic summit.

Police set up barricades and checkpoints within an 800-metre radius of the ExCel Centre in East London's Docklands district as leaders of the G20 met to discuss the global financial crisis.

As the summit meetings began Thursday, about 400 protesters gathered at the barricades near the ExCel Centre, officials said.

Anyone without proper accreditation was being turned away, police said.

A protester keeps watch on various groups as police control the area near the ExCel centre in London on Thursday. (Tom Hevezi/Associated Press)

"I'm here because part of the world's economic troubles can be helped if, instead of spending billions on weapons, we used it to help the poor," said Hazel Barkham, a hospital chaplain, standing near the barriers.

About 100 police officers also continued to guard London's financial district, including the London Stock Exchange, where more than 4,000 demonstrators had gathered Wednesday.

About seven groups were staging small protests in the city's financial district on Thursday, some playing a life-sized game of Monopoly.

"It is fairly obvious the G20 are the global financial elite," protester Clare Smith told reporters. "Meanwhile, the poor are getting poorer. And that has even started to show in this country, but has obviously been going on across the world for some time."

French daredevil Alain Robert scaled a London insurance building to unfurl a banner calling attention to climate change before being led away by police.

Protesters play a giant game of Monopoly outside the London Stock Exchange in London on Thursday. (Sang Tan/Associated Press)

By midday Thursday, about 400 anarchists were gathering near the Bank of London and were confronting police regarding a death at the rally in the financial district Wednesday.

Dozens arrested

More than 100 people have been arrested since Wednesday amid demonstrations that included anarchists, anti-capitalists, environmentalists and anti-war campaigners.

At least one person was reported to have died Wednesday. The unidentified man collapsed at the rally, but the cause of death has not been released.

Riot police and helicopters were heading to the area where the anarchists were gathering, said CBC correspondent David Common.

By Thursday afternoon, the arrests included 16 for violent disorder, 11 for possessing police uniforms and 13 for public order offences, police said.

Two people were also arrested for having weapons, two for aggravated burglary and one for arson — all in connection with a group of demonstrators who broke windows and stormed the Royal Bank of Scotland on Wednesday.

Police said other charges include assault on police, theft and drug possession.

The rally turned violent in some areas and the situation appeared to escalate after police cordoned off the district for part of Wednesday.

Images from the scene showed police waving batons, dispersing clouds of tear gas and pepper spray, with some protesters left with bloodied heads.

Police said several officers were injured during the rally.

The security for the summit is one of the biggest efforts in Britain's history, according to officials, and is aimed at protecting businesses, the Bank of England, the London Stock Exchange and other financial institutions.

With files from the Associated Press