Liberal Democrat leader Nick Clegg said Thursday's milestone should act as a warning to Britain against "blindly" following Washington's lead.

"On this anniversary of the greatest strategic failure in British foreign policy in the post-war period, both Labour and the Conservatives must learn from their fateful decision to back George Bush's invasion of Iraq," he said.

"Never again should the objections of millions of British citizens be so lightly ignored. Never again should we blindly follow instructions from the White House. Never again should we do such untold damage to the international rule of law.

"It is now time to recast British foreign policy in Britain's interests, not those of Washington, starting with an early and full withdrawal of all remaining British troops in Iraq."

The call comes a day after thousands of anti-war protesters rallied to demand Britain withdraw its troops from both Iraq and Afghanistan.

Activists gathered in Trafalgar Square, London, for a march on parliament organised by the Stop the War Coalition - one of a number around the world.

And they are encouraging local groups across the country to observe a minute's silence on the March 20 anniversary of the start of the controversial military action.

Britain still has about 4,200 troops in Iraq, almost all based at the airport in Basra since the handover of the last province to Iraqi control in December.

The Government hopes to reduce that by another 2,000 but no timetable has yet been set for further withdrawals.