An impromptu protest by Tamils as the Prime Minister toured northern Sri Lanka overshadowed the grand opening of the Commonwealth Heads of Government Meeting today.
Davod Cameron travelled to the town of Jaffna to highlight alleged human rights abuses in and after the country's bloody civil war.
At one point he found his convoy mobbed by dozens of Tamil protesters brandishing photos of missing or dead relatives.
It set the tone for an apparently terse meeting with Sri Lanka's President Mahinda Rajapaksa at which he raised his concerns.
ITV News' International Editor Bill Neely reports:
Mr Cameron described the stories he heard in Jaffna as "harrowing" and said the experience "will stay with him".
He visited a welfare centre in the town as well as meeting journalists who were risking their lives to run a daily newspaper.
Countering criticism for not boycotting the summit of Commonwealth leaders, Mr Cameron said he believed he could do more good by "shining a spotlight" on problems in the country.
Back in the capital Colombo, the Prime Minister held an hour-long meeting with President Rajapaksa at which he planned to call for an investigation into alleged human rights abuses at the end of the civil war.
Downing Street said Mr Cameron "pressed his points very directly and robustly" but did not expand on what agreement, if any, had been reached.
President Rajapaksa strongly defended the bloody end to the civil war in his address at the opening ceremony, telling dignitaries:
Speaking at a grand dinner to open the three-day summit, Prince Charles steered clear of controversial topics, praising the Commonwealth and saying that he felt he had grown up in a "family of nations".
As many as 40,000 civilians are estimated to have died in the final months of the regime's 26-year fight with Tamil Tiger separatists, according to the UN.