Hundreds of Tamil protesters have rallied throughout the night outside Parliament demanding the government acts to end war in Sri Lanka.
Rescue boats are on standby amid fears demonstrators will jump en-masse from Westminster Bridge if they cannot speak to Prime Minister Gordon Brown.
The protest is creating traffic problems, with 900 people still at the rally - numbers peaked at about 3,000.
A man is in hospital after jumping into the Thames. No-one has been arrested.
Illegal protest
The UN says 150,000 people are trapped in Sri Lanka's northern war zone and Tamils claim they are victims of human rights abuses.
Sri Lanka's government has rejected calls for a ceasefire with the Tamil Tiger rebel group, which wants independence.
These are people who have relatives and friends in Sri Lanka; people who have lost brothers and fathers and sisters
Suren Surendiran
British Tamils Forum
In pictures: Tamils' London rally
A Metropolitan Police spokeswoman said the illegal protest, which has been going on outside the Palace of Westminster since 1600 BST on Monday, was peaceful.
"Police are continuing to attempt to negotiate with the organisers of the demonstration to facilitate their protest and to minimise disruption to all of London's communities," she added.
She said Westminster Bridge and some surrounding roads had been closed.
Participants failed to give notice of the demonstration and have been waving flags showing the emblem of the Tamil Tigers, which is banned in the UK as a terrorist organisation.
Despite this, the BBC's Andy Moore said early on Tuesday that officers had been allowing people to join the crowd in the hope they would disperse later.
He added: "They are here for the long term, the protesters say. There are families covered with sheets and blankets and children sleeping, despite the noise."
Four RNLI lifeboats, supported by police and fire launches, remain at the scene after the Tamils had threatened to leap into the Thames.
'Stop genocide'
At one stage, as many as 3,000 demonstrators had gathered in the shadow of Big Ben, forcing the closures of Westminster Bridge and the Westminster Underground station.
Suren Surendiran, of the British Tamils Forum, said the protest had been led by students, but that people of all ages from across the UK had joined in.
"These are people who have relatives and friends in Sri Lanka; people who have lost brothers and fathers and sisters," he said.
"As British citizens, they want the UK government to act. Rather than just calling for a ceasefire, they should go to the UN security council to demand one."
Many of the protesters are carrying banners with slogans such as "Stop Sri Lanka's Genocide of Tamils", chanting or banging drums.
Rescue boats are on standby amid fears demonstrators will jump en-masse from Westminster Bridge if they cannot speak to Prime Minister Gordon Brown.
The protest is creating traffic problems, with 900 people still at the rally - numbers peaked at about 3,000.
A man is in hospital after jumping into the Thames. No-one has been arrested.
Illegal protest
The UN says 150,000 people are trapped in Sri Lanka's northern war zone and Tamils claim they are victims of human rights abuses.
Sri Lanka's government has rejected calls for a ceasefire with the Tamil Tiger rebel group, which wants independence.
Suren Surendiran
British Tamils Forum
In pictures: Tamils' London rally
A Metropolitan Police spokeswoman said the illegal protest, which has been going on outside the Palace of Westminster since 1600 BST on Monday, was peaceful.
"Police are continuing to attempt to negotiate with the organisers of the demonstration to facilitate their protest and to minimise disruption to all of London's communities," she added.
She said Westminster Bridge and some surrounding roads had been closed.
Participants failed to give notice of the demonstration and have been waving flags showing the emblem of the Tamil Tigers, which is banned in the UK as a terrorist organisation.
Despite this, the BBC's Andy Moore said early on Tuesday that officers had been allowing people to join the crowd in the hope they would disperse later.
He added: "They are here for the long term, the protesters say. There are families covered with sheets and blankets and children sleeping, despite the noise."
Four RNLI lifeboats, supported by police and fire launches, remain at the scene after the Tamils had threatened to leap into the Thames.
'Stop genocide'
At one stage, as many as 3,000 demonstrators had gathered in the shadow of Big Ben, forcing the closures of Westminster Bridge and the Westminster Underground station.
Suren Surendiran, of the British Tamils Forum, said the protest had been led by students, but that people of all ages from across the UK had joined in.
"These are people who have relatives and friends in Sri Lanka; people who have lost brothers and fathers and sisters," he said.
"As British citizens, they want the UK government to act. Rather than just calling for a ceasefire, they should go to the UN security council to demand one."
Many of the protesters are carrying banners with slogans such as "Stop Sri Lanka's Genocide of Tamils", chanting or banging drums.
