LONDON, June 18 (AFP) - London police mounted a major operation on Friday ahead of expected mass demonstrations against capitalism in the City of London financial district to coincide with the opening of the G8 summit in Cologne.

Police sources said that as many as 2,000 men might be called on to handle up to 10,000 protestors.

The fortified "ring of steel" tightened around the City, as roadblocks that were erected during the hey-day of the Irish Republican Army (IRA) bombing campaign in the early 1990s were reactivated.

Queues of traffic snarled the busy access roads to the business district.

"The road blocks are always here, but they wouldn't necessarily be manned. We are monitoring the flow of people into the City," said an officer on one entry point into the City.

"We are expecting a long day of it."

The City police, which is coordinating a joint operation with the Metropolitan Police and the Transport Police, said that all of its 800 policemen were on duty for the day. All police leave has been cancelled.

"It is a major operation today," a spokesman said.

He added that the force did not have enough wagons to transport its manpower and had been forced to borrow vans from the Kent Police outside London.

"We have got enough resouces to deal with anything that may happen," a Metropolitan Police spokesman said.

The day of protest started with a procession of 250 cyclists, according to police, riding through the City blowing whistles.

Several protest groups including environmentalists, anarchists and proponents of debt relief have called a day of protest to coincide with the start of the G8 summit in Cologne.

"The huge gathering is part of an international movement intending to spotlight the links between economic globalisation, poverty and the destruction of the Earth's environment," said Reclaim the Streets environmentalists.

They were due to stage a "Carnival against Capital" somewhere in the heart of the City later in the day.

The demonstration was part of a coordinated day of protest planned in 41 countries around the world.

Campaigners were expected to hit banks and targets of specific gripes, including McDonalds burger bars and Smithfields meat market in the heart of the City.

The police have seen this all before, notably the "Stop the City" demonstrations of 1983 and 1984.