By Michael Holden

LONDON (Reuters) - Police in riot gear moved in on a central London building where activists had planned an anti-G8 protest through the British capital on Tuesday before next week's summit of world leaders.

The demonstrators had said they would stage a "Carnival Against Capitalism" across London to kick off a week of action before Britain hosts the meeting of the Group of Eight leading industrialized nations at a golf resort in Northern Ireland.

Some of the world's biggest hedge funds, private equity firms and banks have warned their staff to take precautions in the event of disruption after similar protests in recent years led to violent clashes with police, vandalism and buildings being temporarily occupied.

However, hours before Tuesday's protest was due to begin, police descended on premises just off Regent Street in London's fashionable Soho district where the StopG8 group had been meeting before the demonstration.

More than 100 officers in riot uniforms formed a cordon, trapping the gathering protesters in a tight area around Beak Street as police helicopters hovered above and reinforcements waited in vans.

"There's a warrant being executed there," a London police spokesman said. "Our protest liaison teams are actively working to engage with those wishing to protest so that we can facilitate peaceful protest."


Last month, StopG8 issued a map of 100 potential targets for people to "show their anger", identifying offices of financial organizations such as banks, hedge funds, defense manufacturer BAE Systems and mining and energy companies including ArcelorMittal and BP.

The list includes hedge funds Man Group and Paulson, private equity firm Blackstone, banks such as Citi and Barclays and embassies including those of Saudi Arabia and the United States.

"Carnival will go ahead despite cops at Beak St. Don't let them intimidate us! See you 12 noon Oxford Circus and Piccadilly Circus," StopG8 said on Twitter.

The group, which describes itself as an openly anti-capitalist network "made up of autonomous groups and individuals", had refused to cooperate with police, meaning it was not clear how many people would attend, or where they would focus their attentions.

StopG8 has called for supporters to meet at noon at two locations in the heart of central London's main shopping district, Oxford Circus and Piccadilly.

One banker working for an international firm with offices in central London said the staff had received an email indicating some 500 people would attend the protest.

The email said there would be enhanced security checks at its building as it was a possible target.

One large hedge fund, which asked not to be identified, said it had advised its staff to be especially alert to the protests.

"We're clearly aware of it," it said. "Our guidance is to be careful and don't draw attention to yourself as obviously being a hedge fund manager. We're not expecting a riot on our doorstep."

Recent demonstrations against the British government's austerity measures have been marred by rioting anarchists, while many Britons angered by bank bailouts and bonuses during tough economic times blame the financial sector.

In 2009, police made more than 100 arrests after protests by tens of thousands of people to coincide with a G20 economic summit in London turned violent.

There were also clashes between anti-globalization demonstrators and riot police in Scotland when Britain last hosted a meeting of G8 heads in 2005.

(Additional reporting by Mark Anderson, Kylie MacLellan, Anjuli Davies and Laurence Fletcher; Editing by Angus MacSwan)