The tamils were in a bus which was stopped by the military force. Besides, a news team from the UK's Channel 4 TV was stopped from going to a former war zone in the north of Sri Lanka.
About 250 pro-government demonstrators blocked the train carrying the news team at Anuradhapura. The channel is accused of supporting LTTE. They had made a documentary of the Eelam war -No fire zone- Sri Lanka's killing fields- which had won awards.
The UK Foreign Secretary William Hague slammed the incident. He is in Colombo for the summit.
"Concerned to hear of incident with @Channel4News," Mr Hague tweeted. "Have raised with Foreign Minister [GL] Peiris."
Contrary to what the Lankan government had hoped, the summit has its focus on the island country's human rights record. Government wanted to highlight the country's post-war revival.
Indian Prime Minister Manmohan Singh and his Canadian counterpart Stephen Harper have chosen to stay away from the summit, so has Mauritian Prime Minister Navin Ramgoolam.
The final war in Sri Lanka in 2009 ended the civil war which lasted for more than 25 years. Now the government is facing allegations of war crimes.
(With agency inputs)