Police arrested 57 people as the first wave of G8 protests hit London yesterday. Officers carried out a major operation this morning to clear the central London HQ of the anti-G8 movement before their planned march even got underway.

Nearly 1,200 officers were mobilised across London and hundreds swooped on a squat in Soho, tearing down the doors and removing around 40 people occupying it. They also trailed a protest through the West End, making further arrests.

It was planned as the first day of a week of protests against next week's G8 meeting. Anti-capitalist demonstrators identified around one hundred targets across the West End, including banks, hedge funds and even nightclubs they said were symbols of wealth. Instead, it was the police who struck first.

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They searched the Soho building and the squatters they found there, most of whom were subsequently allowed to go free. Police said they made the pre-emptive strike because they feared that the protesters had weapons and were planning to bring serious disorder to central London.

But anti-G8 demonstrators said they had squatted the former police Section House on Beak Street in Soho to protest against the number of empty properties in London and one told The Independent he saw a police officer punch a protester in the face as the building was cleared.

Riot police swooped at around 10am, forming cordons to seal off the building from the surrounding streets. They brought tools in to cut down the doors after protesters barricaded them. People working nearby, who were told not to leave their offices while the operation was carried out, hung out of their windows to get a better view.

Demonstrators hung banners out of the windows, one of which read: "No Pasaran". Some went on to the roof to watch police progress but were soon replaced there by officers. At around 3pm, the first demonstrators began to leave the building as officers moved in. Others, however, continued to resist. Television footage showed police officers in climbing gear trying to secure the roof, bundling a protester who sprang on to the rooftop on to his front.

Police said that at least one of the arrests was made on suspicion of possession of an offensive weapon and at least one more was on suspicion of criminal damage. Police said another person was arrested over an alleged assault on a police officer. But protesters claimed that police assaulted them.

After leaving the building, one squatter - who did not want to be named - said: "There were people on the balcony trying to communicate with the police, asking for time to make a decision because they were worried about homelessness. The police officer said that he would go and make a decision on that. Two minutes later, they came with axle-grinders.

"There was quite a lot of activity but I was just behind the guy who got hit. I was trying to move back and they grabbed him by the scruff of his neck and punched him square in his face and split his lip open."

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As a police helicopter whirred above, another demonstrator - who gave her name only as Jasmine - said she left voluntarily when police got into the building. She added: "They managed to cut through our barricading…people tried to resist and there was a little violence. Everyone is safe.

"I wasn't going to leave voluntarily and then it got to the point that they were in the building and I thought 'no one's going to hold it down now they're in'."

At the same time, another group of around 200-300 anti-capitalists marched through the West End, while a large police escort trailed them. They visited some of the target sites, which featured on a map posted online last month, and were met everywhere with lines of police. Following police advice to West End businesses to take precaution, the Ritz pulled down its shutters. Officers were also standing outside Fortnum and Mason, which was the target of a previous protest in 2011.

A police spokesman said the arrests were on suspicion of "various offences including violent disorder, possession of pointed/bladed articles and a variety of public order offences". Police "categorically" denied a Taser was used in clearing the building after reports surfaced online one had been discharged.

Video: Police and G8 protesters clash in London