Demonstrators have tried to storm the British high commission in Sri Lanka's capital Colombo over Tamil protests during President Rajapaksa's UK visit.
A Sri Lankan government minister led the rally outside the building, accusing Britain of supporting Tamil Tiger separatists.
Mahinda Rajapaksa's speech to the Oxford Union was cancelled this week because of Tamil protests.
Sri Lanka denies war crimes during its defeat of the Tamil Tigers last year.
Both sides have been accused by human rights groups of crimes against humanity during the final phase of the 26-year insurgency, which ended in May last year.
'British Naked With Tigers'
Hundreds of flag-waving protesters tried to push down police barricades outside the British high commission on Friday and advance on the building, but they were held back by police.
They carried placards reading "Is British Law a Tiger Law", "British Naked With Tigers", "British Shame On You" and "Tiger Puppets Go Home".
They were led by Housing Minister Wimal Weerawansa, who accused the UK government of silently supporting sympathisers of the Tamil Tigers by allowing protests in London against President Rajapaksa's visit this week.
He said the "British white colonial government" was accusing Sri Lanka of war crimes in an effort to tarnish the country's image.
"By allowing and supporting pro-LTTE [Liberation Tigers of Tamil Eelam] protests, the British government not only threatened Sri Lanka's president, they also got the revenge for [us] ending the war and bringing peace to this country, which they never wanted to happen," Mr Weerawansa added.
Meanwhile, President Rajapaksa was cheered by large crowds of supporters as he returned from the UK, touching down at the international airport in Katunayake on the west coast.
He had been due on Thursday to address the prestigious Oxford Union debating society, which has hosted speakers such as Michael Jackson and the Dalai Lama, but the event was cancelled because of security concerns.
The union said in a statement that "due to the sheer scale of the expected protests, we do not feel that the talk can reasonably and safely go ahead as planned".