Tamil protesters are staging a demonstration outside the Mansion House in the City of London.
The Sri Lankan President Mahinda Rajapaksa had been due to address a Diamond Jubilee Commonwealth Economic Forum, but the morning session of the event has now been cancelled.
The protesters accuse Rajapaksa of running a dictatorial regime which represses the Tamil people.
A spokesman for Tamil Solidarity said:
"This butcher and his dictatorial regime is still brutally repressing the Tamil-speaking people and trampling democratic rights. Britain has enough war criminals without Rajapaksa, with the blood of hundreds of thousands on his hands, 40,000 in the last weeks of the war alone, being invited for the Jubilee."