Police leave has been cancelled, shops and banks have been bordered up and the sun is out as London prepares to welcome the leaders of the 20 most powerful countries on Earth.

A huge £7.5m security operation by 5,000 officers has been mounted in the face of potentially violent protests against the G20 by anti-capitalist, anti-globalisation and environmental activists.

Previous meetings between world leaders and bankers in Seattle and Genoa have seen riots explode on the streets.

Police have said the effects of the global downturn make the nature of these protests "unprecedented".

Tank stopped - arrests

While most protests are expected to be peaceful, in a sign of things to come in the next few days an armored tank with half a dozen army suited occupants was stopped by City of London police.

Police searched the vehicle from top to bottom before releasing the group.

Four people were arrested yesterday, all charged with assualting a police officer and another for possession of a knife.

One potential flashpoint is the Bank of England where protesters plann to "storm" the building and mount tents outside the building.

US embassy protest

A potentially volatile meeting is the Stop the War Coalition who will gather outside the US Embassy at Grosvenor Square at 2pm and then march on Trafalgar Square.

The Financial Fools' Day protesters have just left Cannon Street, Moorgate, Liverpool Street and London Bridge stations to march to the Bank of England. The huge network of allys and back streets make the marshelling of the event a nightmare for police.

At 12.30 Climate Camp groups who claim a day of "spectacular action" are converging on European Climate Exchange and at 5pm a protest will be held on the Mall outside Buckingham Palace.

On Saturday tens of thousands of people marched through London demanding action on poverty, climate change and jobs.

Umbrella group

Activists in the umbrella group, G20 Meltdown, representing 20 groups are planning a day of direct action.

They are appealing those effected by the economic downturn to join what they call a "Financial Fools' Day" targeting the banking elite.

Cops fear the reemergence of hardcore protesters from the early 90s like the Wombles as well as experianced European anarcists.

Operation Glencoe - tactics

Codenamed Operation Glencoe, six police forces are working to ensure peaceful protest and a safe conference at the Excel Centre where world leaders are meeting.

Commander Simon O'Brien has warned that some protests may be "hijacked" by extremists but officers have been heavily monitoring internet chatter to disrupt and prevent renegade groups.

Police have denied that tasers will be deployed.

The Public Order Act will be in force rather anti-terror laws and arrests:

"Clearly we have tactics that we have planned and used before in the past and we will be using those sort of planned tactics again today, if things move forward in that direction."