The City of London Corporation's Judith Mayhew believes the protest is "sinister"

The event, billed as June 18 or J18, coincides with the summit of the leaders of world's seven richest countries in Cologne, who are expected to announce $100bn in debt relief.

Activists around the world promised to take similar action. In Cologne, police expect up to 100,000 demonstrators, including Irish rock star, Bono, and Live AId organiser, Bob Geldof, to form a human chain around the West German city on Saturday.

Protests against capitalism are already taking place in developing countries like the Philippines.

As trading started in London, 300 cyclists disrupted traffic, carrying banners with slogans like "Money Kills".

Activists also daubed pink paint on the door of the London Metal Exchange.

Ring of steel

In Threadneedle Street, home to the Bank of England and arguably the heart of the City, traffic was brought to a standstill.

Police precautions added to the chaos on the roads.

The City's own police force has cancelled all leave and plans to deploy as many as 2,000 men to confront up to 10,000 protesters.

The police resurrected its "ring of steel" around the square mile, which was first introduced during the last bombing campaign of the IRA.

Made up not of steel, but plastic road blocks designed to monitor incoming traffic, the protective ring snarled up 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.

No take-aways

The protesters are circulating maps of the Square Mile, showing targeted banks and companies. J18 activists promised that there would be no violence against people, but did not rule out property damage.

One mooted action is putting super glue into the door locks of City banks, or disrupting computer systems and occupying offices.

The demonstrators said they would dress up as bankers and try to sneak into banks. That actually could prove to be the wrong strategy, as Friday is "dress down day" in many banks, where three-piece suits are replaced with "smart but casual" dress.

Businesses have been advised to increase security, and many banks are trying to spare their staff any confrontation with the demonstraters.

During lunchtime, the City's numerous bars and sandwich shops are normally swarming with workers grabbing a take-away meal.

On Friday, though, many companies have organised for packed lunches to be given to staff at their desks.