London's Metropolitan police have mounted an unprecedented security operation as tens of thousands of protesters converged in London on Wednesday for a series of demonstrations during Thursday's G-20 summit to be attended by world leaders, including Prime Minister Manmohan Singh.

Security officials fear that peaceful demonstrations may be hijacked by elements intent on using violence and causing disruptions. Demonstrators have reached London from various parts of the UK as well as nearby countries n Europe.

At least four 'carnival parades' are expected to culminate in a major demonstration outside the Bang of England, to highlight job cuts and recession in recent months.

The G-20 Meltdown website says the focus will be on 'thrusting into the very belly of the beast: The Bank of England'. It adds: "Their tax-dodging, bonus-guzzling, pension-pinching, unregulated free market world's in meltdown."

Demonstrations are planned in London on Thursdy and anti-war groups are expected to gather outside the US Embassy and at Trafalgar Square. More protests are expected near the G-20 conference centre on Thursday.

London's Metropolitan Police is in the forefront of the security operation that includes personnel from five other police forces. The security operation is reported to cost 7.5 million pounds.

Commander Simon O'Brien said the police monitoring of the Internet showed that some people were planning to come to London to 'commit acts of crime and disorder'.

He said: "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."