Police made four arrests on Tuesday as the protesters were hemmed in by more than 100 officers. Rescue boats have been put on alert along the river Thames amid fears the protesters -- flying the flag of terrorist Tamil tigers -- may leap en masse from Westminster Bridge if they are not allowed to speak to British Prime Minister Gordon Brown.

Carrying banners reading "Stop Sri Lanka's Genocide of Tamils" and "Rwandan Genocide Repeats in Sri Lanka", the protesters waved flags of the Tamil Tiger rebels who face a threatened final assault by government forces.

One person jumped off the bridge on Monday evening after up to 5,000 protesters converged on the Parliament on Monday afternoon to protest over the Sri Lankan government's offensive against the rebels.

Rescuers plucked the man after he jumped in about 6 pm (GMT). He was taken to a hospital and the police said his condition is not serious. Organisers of the demonstration failed to give notice -- required by law -- and protesters have been carrying the emblem of the Tamil Tigers, banned in the UK and the European Union as a terrorist organisation.

A police spokeswoman said, "Police are continuing to attempt to negotiate with the organisers of the demonstration to facilitate their protest and to minimise disruption to all of London's communities."

Sri Lanka has rejected international calls for a ceasefire, claiming it is on the verge of defeating the separatist Tamil Tigers. With the remnants of the Tamil Tigers scattered after most of their bases having fallen, the Lankan troops are now engaged in man-hunting combat looking to finish off or capture the Tiger leaderships, including its Supremo V Prabhakaran.