Four lifeboats were scrambled after some of the protestors reportedly threatened to throw themselves into the nearby River Thames. One person did enter the water and was taken to hospital, but was not thought to be hurt.
Sri Lankan President Mahinda Rajapakse on Sunday said the military was close to wiping out the Liberation Tigers of Tamil Eelam after decades of bitter fighting for an independent homeland.
Rajapakse has rejected international calls for a truce which the government has argued would only let the rebels regroup.
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.
At the height of their power in the mid-1990s, the Tigers controlled more than a third of Sri Lanka.
Objects including cans and a stick were thrown as a few hundred protestors were rounded up on Parliament Square, after being cleared away from Westminster Br