Activists of Ravana Balaya demonstrated in front of the UK's embassy here demanding Britain must investigate the crimes of its own troops in British Ceylon in 1818.
In a communique handed over to the embassy, Ittakande Saddatissa, a Buddhist monk and Ravana Balaya chief, said, "We have found very clear evidence to prove as to how British Army executed captured freedom fighters without going through a proper judicial process.
Cameron, on the sidelines of the summit, had warned Sri Lanka that his government would work with the UN Human Rights Council to set up an international inquiry if the country failed to address rights concerns by March next year.
Rajapaksa had rejected Cameron's warning, saying countries should not "dictate" to Sri Lanka.