"More than 50,000 were at the demonstration," a spokesman for London's Metropolitan Police said.
"There has been one arrest for possession of an offensive weapon.
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The spokesman said the protest was largely peaceful and good humoured.
The rally began at Atterbury Street, by the Tate Britain gallery and passed along the north bank of the River Thames to Temple Place.
Demonstrators banged drums and waved flags and banners along the route.
The protesters delivered a petition to British Prime Minister Gordon Brown's Downing Street office, urging the government to persuade Sri Lankan forces to declare a ceasefire.
The Sri Lankan military has been battling cornered Tamil Tiger rebels in the north of the island.
The Red Cross has been trying to negotiate the evacuation of more civilian wounded from the conflict zone.
The military says it is in the final phase of a year-long offensive to wipe out the Tiger guerrillas and end Asia's longest-running ethnic confl