Hundreds of Tamil activists have gathered on Pall Mall in protest against the Sri Lankan president as he attends a lunch for Commonwealth leaders with the Queen.
Mahinda Rajapaksa, who has been accused of presiding over human rights abuses, cancelled a speech he was due to give prior to the lunch, amid fears it would spark demonstrations in the capital.
According to the Evening Standard, up to 3,000 pro-Tamil protesters turned out on the streets of London on Wednesday morning ahead of the president's appearance at the lunch.
Rajapakse was seated on the table directly to the Queen's left with Babli Sharma, wife of the Commonwealth Secretary- General, Namibian President Hifikepunye Pohamba and his wife, and New Zealand Prime Minister John Key and his wife.
The 11 tables were named after flowers, with the Queen seated on the Golden Wattle table.
Sharma welcomed the guests, saying: "It gives me great pleasure to welcome you all on this very special day in the history of the Commonwealth."
The guests were served a Brie and avocado terrine followed by wild sea bass then an apple crumble souffle, apple pie ice cream and caramelised apple.
The meal was accompanied by South African wines.
The Queen shaking hands with Sri Lankan president Rajapaksa
After she arrived at the lunch The Queen spent a brief moment with Rajapakse and appeared to fleetingly shake hands with him as she met guests at a reception in the Blenheim Saloon inside Marlborough House.
The president's speech was cancelled by organisers the Commonwealth Business Council (CBC) on Tuesday evening.
Demonstrators burnt an effigy of Rajapaksa outside Marlborough House
A spokesperson for the police told Channel 4 News the CBC had "decided it was not in their interest to stage the event" due to the extent of protection required amid expected protests over the treatment of Tamils.
Demonstrators gathered outside Marlborough House where President is due to lunch with Queen
Tickets for the Commonwealth Economic Forum special meeting had been pre-sold at £795 each, plus VAT.
Sen Kandiah, founder of the British Tamil Forum, told the Guardian "common sense" had prevailed.
"There is now enough evidence that allegations of war crimes in Sri Lanka lead directly to the president himself. That is why British government officials are reluctant to meet him. He is not welcome here.”
Amnesty International said in March 2012 human rights abuses were rife in Sri Lanka, with violations going without punishment.
A Civil War between Tamil Tigers and the Sri Lankan army ended three years ago after a quarter of a century. However there remain allegations of torture and mistreatment of Tamils in Sri Lanka, with the Guardian reporting Britain is deporting Tamils who will face torture in Sri Lanka.