UK television journalists who have reported on war crimes allegations in Sri Lanka have been stopped from reaching the north of the country by protesters who blocked their train.

The Channel 4 news team was in Sri Lanka to cover a Commonwealth summit that will start on Friday.

It has has already been marred by concerns about human rights abuses and the leaders of Canada and India are staying away.

Around 100 protesters blocked the train on which the news crew was travelling, trying to get to the north of the island, the scene of the government's war against Tamil Tiger rebels that ended in 2009.

"Five hours north of Colombo, in the city of Anuradhapura, a large mob of pro-government demonstrators met the train and then blocked the tracks, preventing the train - which had hundreds of passengers on board - from continuing," Channel 4 said.

Among the journalists was Callum Macrae whose documentary - No Fire Zone: The Killing Fields of Sri Lanka - was released last week.

The documentary shows video of a Tamil Tiger television presenter being captured and killed in what the filmmaker said was a one of a "pattern of war crimes" committed in the final days of the war

The military called the film an attempt to discredit Sri Lanka before the Commonwealth summit that brings together heads of state and government from around the world.

Police said the Channel 4 team decided to return to the capital after their journey was blocked.

Channel 4 Foreign Affairs Correspondent Jonathan Miller tweeted: "Escorted by police off train through yelling mob. We are now driving south under police escort."

The journalists also said they had faced protests on arrival at the airport, an area where demonstrations are never normally allowed, and outside their hotel in central Colombo where protests have been banned during the Commonwealth summit.

"It ... appears that the ban only applies to those who would demonstrate against the government," Mr Macrae said.

"Government supporters seem able to organise demonstrations at will and the police show no desire to stop them."

Local media reported that President Mahinda Rajapaksa had invited the journalists to visit any part of the country during their stay to see the progress made since the war.

Tens of thousands of civilians were killed in 2009 in the final months of the war, a UN panel has said, as government troops advanced on the northern tip of the island controlled by Tamil rebels fighting for an independent homeland.

The panel said there were "credible allegations" that Sri Lankan troops and the Tamil Tigers both carried out atrocities but said the government was responsible for most of the deaths.

Canadian Prime Minister Stephen Harper has declined to attend the summit because of concerns over abuses. India's Manmohan Singh also pulled out.

The UN Human Rights Commission has passed two resolutions urging Sri Lanka to investigate the war crimes allegations, something Mr Rajapaksa's government has rejected.