Thousands of Tamils blocked streets outside the Houses of Parliament today, accusing the Sri Lankan government of disregarding civilians caught up in its war against Tamil Tiger rebels and demanding a ceasefire.
Large numbers of police were deployed to contain the peaceful demonstration after it spilled out from the square in front of Parliament to block several major streets.
Expatriate Tamils have been demonstrating around the world, but London has been a focus as many Tamils blame Britain, as the former colonial power, for denying them a homeland.
Police asked the protesters to unblock roads, but around 2,000 continued to block a main thoroughfare, police said.
Members of Britain's Tamil population - estimated to number around 200,000 - have been demonstrating for two weeks.
Sri Lankan government troops have encircled Tamil fighters in a small pocket in the northeast of the island, and their shelling has raised concerns for the civilians trapped there.
The government today gave the guerrillas 24 hours to surrender after several thousand civilians managed to flee the war zone. The International Crisis Group said a "humanitarian tragedy" was unfolding in the region, with around 150,000 civilians still trapped.
Angry demonstrators in London held aloft banners denouncing the conflict as government-led genocide.
"Hundreds of Tamil people are dying every day, we can't let this go on," said Sayan Selvaratnan, 29, who was given asylum in Britain eight years ago and whose parents remain in the north of Sri Lanka.
Among the Tamil demonstrators is 28-year-old Subramanyam Parameswaran, who has been on hunger strike for 14 days and says he will starve himself to death unless his demands, including a ceasefire and UN intervention, are met.
Protest leaders said they were determined to keep the demonstration going for as long as necessary. Amid their fervour, however, there was also misinformation about the war.
Virtually all those spoken to accused the government of using chemical weapons against the Tamils, although there is no independent evidence of their use.
One man said 1,000 civilians had been killed today alone, while another a few feet away put it at 5,000.
An impassioned woman said she had heard it was 10,000, after government troops forced civilians to walk through a minefield and then gunned down those who refused to do it. Ten minutes later, she returned to say it was actually 20,000 people.