Police estimated that more than 1,500 pro-Tamil demonstrators turned out on the streets but eye witnesses at Pall Mall said there were closer to 3,000.
The Queen was due to meet Sri Lankan president Mahinda Rajapaksa today at a Commonwealth lunch.
Mr Rajapaksa, who has been accused of presiding over human rights abuses after allegations of war crimes by Sri Lankan armed forces, cancelled an address in the City of London amid concerns about the protest.
He was still expected to attend the Commonwealth lunch for the Queen at Marlborough House on Pall Mall.
The protest was the first major Tamil demonstration in London since a series of massive demos across London three years ago.
Then tens of thousands of Tamil protesters caused huge disruption on central London and made several attempts to storm Parliament, costing Scotland Yard more than £12 million in overtime costs.
Police said today they were monitoring the gathering at the corner of St James Street and Pall Mall, close to where the Queen was attending the lunch.
Channel Four’s foreign affairs correspondent Jonathan Miller tweeted: “Tamil protesters are wielding effigies of Rajapakse hanging from a gallows.”
Mr Rajapaksa was due to give the keynote speech at a special Diamond Jubilee meeting of the Commonwealth Economic Forum on Wednesday morning, but the event’s organisers, the Commonwealth Business Council, stated on its website: “After careful consideration, the morning sessions of the Forum … will not take place.”
A spokesman for Scotland Yard said it had agreed to guarantee the president’s security but the CBC had “decided it was not in their interest to stage the event” because of the extent of the policing required and the likely disruption to the City of London.
Fred Carver, campaign director of the Sri Lanka Campaign, welcomed the news, calling it a “testament” to the campaign.
The protest was also aimed at the Hilton hotel on Park Lane where the president is staying.
There were huge protests in 2009 when the 26-year war in Sri Lanka, which claimed an estimated 70,000 lives, ended with government forces defeating the Tamil resistance.
The protests come after a Sri Lankan man, who was left scarred and suicidal after two weeks of torture, accused the British government of forcibly deporting asylum seekers who are then tortured in Sri Lanka.
The victim told the Guardian newspaper he was tortured over the space of 17 days after being deported from the UK last year.
His torturers accused him of passing on to British officials information about previous beatings at the hands of state officials and other human rights abuses to ruin diplomatic relations between the two countries. (London Evening Standard)