LONDON—Police have forcibly removed anti-G8 demonstrators who barricaded themselves inside an abandoned building, saying they did it because they believed weapons were inside.

Those weapons could have caused harm to public property and officers, a Scotland Yard official said.

The protesters had been squatting in the building for days, readying for a week of protests in London leading up to next week’s G8 meeting in Lough Erne, Northern Ireland.

The demonstrators intended to join the thousands expected from around the world who have gathered for what they call the “carnival against capitalism.”

Meanwhile in St. James’s Square, other demonstrators amassed in front of energy firm BP’s offices, banging drums and waving flags. Outside Lockheed Martin, some chanted “war criminals.”

Police — and media — chased protesters most of the day through the streets of downtown London, from Oxford Circus and Piccadilly Circus to the Charing Cross police station. As of late Tuesday, 32 were arrested.

The Stop G8 movement warned earlier that it would target hedge-fund groups, major banks and other multinational corporations headquartered in London.

Protests are expected during G8 summits, which is why they are held in remote, hard-to-get-to locations. When Canada hosted the G8 meeting in 2010, it was held in Huntsville.

This time, demonstrators are planning to voice their concerns in London and Belfast.

At least 200 police in full visors and helmets kept pedestrians and demonstrators away from the four-storey former police building on Beak St. where the squatters had taken refuge.

Police dressed in climbing gear staged a dramatic takedown of one young man who emerged from the occupied building and seemed to try to run off the roof.

Tammy Samede stayed in the building on Monday evening. Samede had come to take part in planned demonstrations at Piccadilly and Oxford Circus. She told the Star the police arrived Monday afternoon and began to surround the building.

“I only came in last night and this morning, about an hour ago, I was out having a coffee and a cigarette and the police swarmed onto the roof, they came over the roof,” she recalled Tuesday.

She noted it was ironic the protesters were occupying a former police building. “Yeah, it is one of theirs,” she scoffed.

The police action snarled central London, as dozens of police vans, full of officers in riot gear and canine units, parked along the streets. Reports say 1,200 officers were deployed throughout London. Some small avenues off the fashionable Regent St. were completely blocked as traffic came to a halt.

As protesters and interested Londoners gathered behind the police lines to watch the scene unfold on Beak St., demonstrators from inside the occupied building emerged on the roof, some with their faces shielded by red and black scarves. They waved and taunted the police below.

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“Shame on you!” yelled protesters.

The leaders of the world’s most powerful nations are gathering at the remote Lough Erne golf resort on Monday and Tuesday to discuss trade, tax evasion and Syria.