By Shihar Aneez and Ranga Sirilal
COLOMBO (Reuters) - Pro-government protesters blocked a train carrying a British TV crew and the Sri Lankan army stopped activists trying to attend a human rights conference on Wednesday, in an apparent clampdown on free movement ahead of a Commonwealth summit.
Hundreds surrounded the train and some boarded it, a witness said, adding that police made no attempt to clear the crowd. The Channel 4 television news team, which has previously reported on alleged war crimes in Sri Lanka, had to return to the capital Colombo, the TV station and local police said.
Separately, three buses carrying relatives of disappeared ethnic minority Tamils were stopped by the military on their way to participate in a human rights meeting in the capital, witnesses and Amnesty International said.
The Channel 4 team is in Sri Lanka to cover the Commonwealth Heads of Government Meeting (CHOGM) which starts on Friday in Colombo and has already been marred by concerns about human rights abuses. The leaders of Canada, Mauritius and India are staying away.
Around 100 protesters blocked the train carrying the news crew towards the north of the island, the scene of the government's war against Tamil Tiger rebels which 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 on its website.
Among the journalists was Callum Macrae, whose documentary, "No Fire Zone: The Killing Fields of Sri Lanka" showed video of a Tamil Tiger television presenter being captured and killed in what the filmmaker said was part of a "pattern of war crimes" in the final days of the war.
The military called the film an attempt to discredit Sri Lanka before the summit, which brings together heads of state and government from around the world.
The police said the Channel 4 team had decided to return to the capital after their journey was blocked.
"They protested against these journalists travelling to the north ... saying that these journalists were tarnishing the image of Sri Lanka," said police spokesman Ajith Rohana.
Britain's Foreign Secretary William Hague expressed "serious concern" about the incident in a conversation touching on media freedom with his Sri Lankan counterpart, the British government said in a statement.
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 on Monday at Colombo's airport, where demonstrations are never normally allowed, and outside their hotel in central Colombo, where other protests have been banned during the summit.
"It ... appears that the ban only applies to those who would demonstrate against the government," Macrae said. "Government supporters seem able to organize 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 progress made since the war.
Separately, around 100 ethnic Tamils travelling by bus to a human rights conference in Colombo were prevented by the military from proceeding near Anuradhapura.
They say they were prevented from getting to the meeting. The military said they were stopped at a checkpoint and chose to turn back.
Military Spokesman Ruwan Wanigasooriya said the buses were stopped as part of a security operation ahead of CHOGM and they would have been allowed through if they had been patient.
"They must have wanted to go back and create a situation. Not only these vehicles we checked, but all vehicles due to the present situation," he said.
One of the activists told Reuters two buses were told to return after being stopped, adding that a third bus headed to the meeting but was later also forced back.
"Commonwealth leaders must not just turn a blind eye," Steve Crawshaw, director of the Office of the Amnesty International Secretary General, said in a statement about the incident.
"It is notable that the Commonwealth has been shamefully silent throughout this, and has yet to condemn the human rights violations that are still so clearly business as usual for Sri Lanka," he said.
Crawshaw also said Sri Lanka is trying to use the summit to whitewash its "despicable human rights record and hide ongoing abuses under the carpet".
Tens of thousands of civilians were killed in 2009 in the final months the war, a U.N. 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 declined to attend the summit because of concerns over abuses. India's Manmohan Singh and the prime minister of Mauritius also pulled out.
The U.N. Human Rights Commission has passed two resolutions urging Sri Lanka to investigate the war crimes allegations, something Rajapaksa's government has rejected.
(Editing by Frank Jack Daniel and Andrew Roche)