Police officers were caught napping this afternoon after nearly 500 Tamil Tiger protesters occupied streets around the Houses of Parliament.

Demonstrators calling for a ceasefire in Sri Lanka at one point outnumbered officers, spilling through police lines - all approaches to Parliament Square from Westminster Bridge and Whitehall are shut and protesters are blocked in with vans.

The main entrance to the Houses of Parliament was also closed off, to prevent the crowd surging towards the Commons through the Carriage Gates.

Supporting terrorism?

There is speculation several activists could be arrested for supporting a terrorist group - several arrests have been made in previous protests for carrying a flag supporting a proscribed organisation.

Many are openly displaying the emblem of the Tamil Tigers - a banned terrorist group in the UK under the Terrorism Act 2000.

"Bloodbath"

Protesters, including hunger strikers, have been camped in Parliament Square since early April.

The United Nations has described the situation in northern Sri Lanka as a "bloodbath" after reports of heavy civilian casualties at the weekend - Foreign Secretary David Miliband has just returned from the war torn island.

Gordon Brown has sent a special envoy to the United Nations in New York for "urgent" talks on Sri Lanka, a former colony.

The protest for the UK to intervene Sri Lanka's civil war is now entering a fifth week.

One protester said:

"The rise in the human catastrophe has resulted in us being here, we want Briton to act now"

Half of the UK's 300,000 Tamils live in London.

'Noisy but peaceful'

A Metropolitan Police spokesman said:

"At about 10.30am today about 500 protesters in Parliament Square pushed through police lines across the road towards the Palace of Westminster.

The protesters are now sitting in the road between Parliament Square and Whitehall. Police are currently negotiating with the demonstrators to get them back into Parliament Square."

The demonstration was described as noisy but peaceful, with chants urging a ceasefire in Sri Lanka.

Police said 3,000 protesters had taken to the streets of London during the initial demonstrations at the beginning of April, there have been ugly clashed as police moved demonstrators off the roads and activists have thrown themselves into the Thames.

Civil war nearing end

The upheaval comes during the death throws of the Tamil Tiger uprising and a 25 year old civil war in Sri Lanka draws to an end.

The United Nations says 150,000 people are trapped in the northern war zone while Tamils claim they are victims of human rights abuses.

Sri Lanka's government has rejected calls for a ceasefire with the rebels, who want independence - the group is staring defeat in the face after being pushed back onto a narrow peninsular.