LONDON (AFP) - More than 100 riot police on Tuesday surrounded a building in central London thought to be an anti-capitalist squat, as protesters demonstrated in the British capital ahead of next week's G8 summit in Northern Ireland.

An AFP photographer said three police abseiling teams had positioned themselves on the roof of the building, a former police station in the Soho district which is believed to be occupied by protesters.

Television pictures showed officers removing large pieces of plywood from one exit.

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.

Some 70 police in riot gear were at the scene of the protest and there were several scuffles between officers and demonstrators, an AFP photographer said.

Leaders of the world's richest economies including US and Russian presidents Barack Obama and Vladimir Putin are due to meet at Lough Erne in Northern Ireland on June 17-18.

A huge security operation has been launched in Northern Ireland ahead of the summit, with more than 3,500 officers drafted in from the British mainland.

No-fly zones will be imposed over parts of Northern Ireland during the summit and surveillance drones will be used to guard key sites.