The police operation came as around 150 protesters paraded through central London, banging drums in a “Carnival Against Capitalism” organised by the group Stop G8.
Police officers are in a stand-off at a reported squat in Beak Street, a former police station in the Soho district.
The Police Metropolitan officers have told occupants of the building that they were free to leave, but would be subject to searches if they did.
Local residents and office staff have reported about a strong police presence in and around Oxford Street and Regent Street.
The demonstration comes ahead of a planned “Carnival against Capitalism”, organised by anti-G8 group StopG8. Next week G8 world leaders will gather for a two-day summit in Northern Ireland.
U.S. president Barack Obama will also visit parts of Belfast ahead of the summit and police protection of landmark sites across Belfast has been tightened in advance.
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