Police seized Tamil Tiger flags and made a series of arrests outside Parliament today as officers struggled to control a growing demonstration over alleged human rights abuses in Sri Lanka.

Scotland Yard warned it may have to make “a huge number of arrests” as the size of the Tamil rally swelled beyond 1,000 after the demonstration seemed to have been quelled this morning.

Protesters were forced into the centre of Parliament Square by lunchtime as police finally pushed the unauthorised Sri Lankan demonstration off Westminster’s busy roads after almost 24 hours of disruption.

The Metropolitan Police moved in to try to end the overnight protest, triggering minor scuffles and around four arrests.

An unconfirmed number of Tamil Tigers (LTTE) flags have been seized by officers who warned the protesters that it was illegal to carry images representing a proscribed terrorist organisation, under the Terrorism Act 2000.

A Scotland Yard spokesman said: “Numbers are rising and police estimate there is now about 1,000 people gathered.

“Police are continuing to negotiate with community leaders in an attempt to resolve the situation and avoid having to make a huge number of arrests.”

It had appeared that the worst of the demonstration was over. Thousands of Tamil demonstrators blocked Westminster Bridge, Victoria Embankment and the roads around Parliament Square from around 3pm yesterday and Westminster Underground Station was closed this morning as officers attempted to restore order.

At one stage the protest had expanded to about 3,000 men, women and children, some of whom reportedly threatened to leap off Westminster Bridge into the Thames. Four lifeboats were scrambled and one person did enter the water – they were taken to hospital, but were thought not to have been hurt.

Demonstrators chanted “shame on British police” and “we want a ceasefire,” while some lay down on the ground, after brief skirmishes between demonstrators and officers. At least two people were taken away on stretchers, although there were apparently no serious injuries. Carrying banners reading “Stop Sri Lanka’s Genocide of Tamils” and “Rwandan Genocide Repeats in Sri Lanka”, the protesters waved flags of the Tamil Tiger rebels who face a threatened final assault by government forces.

By this morning the rally appeared to dwindled to between 200 and 300, before it grew once more. Objects including cans and a stick were thrown as they were rounded up in Parliament Square after being cleared away from Westminster Bridge.

One demonstrator, who declined to be named, vowed: “We will stay until we have an answer from Gordon Brown ... Our relatives are dying out there and so we will stay here."

As the protesters were clashing with police, David Miliband said the Government was “very concerned” about civilians caught up in fighting in Sri Lanka.

The Foreign Secretary said: “Recent reports suggesting that the Sri Lankan military have now captured all the territory outside the so-called ’no-fire zone’ and that fighting is now going on inside the zone, where the civilian population is concentrated, are deeply worrying.”

“The need for a humanitarian ceasefire is now even more urgent,” Mr Miliband added. “Nothing excuses the reported use of civilians by the LTTE as a human shield . . . But nor does the LTTE’s behaviour excuse any failings by the Sri Lankan government to meet the high standards expected of democratic governments in conflict.”

The United Nations and other foreign aid organisations say as many as 150,000 civilians may still be trapped in the war zone, although the Sri Lankan government insists the figure is less than half that. A UN human rights official repeated calls for a ceasefire today.

President Mahinda Rajapakse of Sri Lanka said the military was close to wiping out the LTTE after decades of bitter fighting for an independent homeland. He rejected international calls for a truce which the government has argued would only let the rebels regroup.

Vijay Mahalingam, 28, one of the protesters from Harrow, said: “We are here to call on the British government and other governments of the world to wake up to the problems in our homeland Sri Lanka.

“Last week, the most powerful men in the world were here in London but there was no mention of the struggle lots of our people have to face on a daily basis.

“Members of my family, some of our grandparents, are stuck in an area in the north-east of Sri Lanka which has been bombed intensively for the last two months.”

He added: “There is no media except the state media so we have no real way of knowing exactly what is going on. There is no phone contact and every day we just hope they have not been harmed."

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