LONDON (AFP) - Riot police on Tuesday forced their way into two buildings in London occupied by anti-capitalists, and clashed with protesters demonstrating in the British capital ahead of next week's G-8 summit in Northern Ireland.

London's Metropolitan Police said nearly 1,200 officers had been deployed and 32 protesters had been arrested.

More than a hundred riot officers surrounded a former police station in the Soho district thought to be an anti-capitalist squat, and after a tense standoff pushed their way inside to escort several people from the building.

Aerial television footage showed a protester being pinned down by officers on the roof, where several police abseiling teams had positioned themselves. A banner reading "What if we smash the G-8?" was hanging from a window.

London's Metropolitan Police said they had "a search warrant relating to intelligence that individuals at the address were in possession of weapons and were intent on causing criminal damage and engaging in violent disorder".

Officers removed large pieces of wood and metal poles from the building.

In east London, police said they had raided a second building following reports of criminal damage. Three people were arrested.

The raids came as around 150 protesters paraded through central London, banging drums in a "Carnival Against Capitalism" organised by the group Stop G-8.

There were several scuffles between demonstrators and dozens of riot police at the scene.

Protesters hung a large banner reading "Make extreme wealth history" onto the famous statue of Eros at Piccadilly Circus.

Another group of demonstrators, who had covered their faces with black scarves, marched with a sign saying: "No borders, no prisons, no capitalism".

There was also stand-off as police prevented protesters from accessing the famous Fortnum and Mason department store.

A spokesman for the Home Office, or interior ministry, said peaceful protest was "a vital part of a democratic society, providing it is conducted within the law".

But she added: "Rights to peaceful protest do not extend to violent or threatening behaviour and the police have powers to deal with any such acts."

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

A massive 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.