Hundreds of Tamil and human rights campaigners have held a demonstration outside Marlborough House in central London.
There were angry scenes as they voiced their opposition to Sri Lankan President Mahinda Rajapaksa's presence at a lunch for Commonwealth heads of state.
The Queen attended the meal along with more than 70 guests, including Prime Minister David Cameron and leaders from across the association of nations.
Mr Rajapaksa was jeered as he swept through the main gate in a Range Rover.
His presidency has been tainted by persistent allegations of war crimes committed by Sri Lankan armed forces.
His car did not carry a flag because of security concerns.