"We want to send a clear message to (Prime Minister) Tony Blair that we and the British people are fed up with the half-truths and evasions on Iraq," said Stephen Tindale, executive director of environmental group Greenpeace, which organised the stunt.

The climbers scaled two fences at the base of the tower, which is subject to some of the tightest security in Britain.

Armed police guard parliament and concrete blocks ring the building to prevent suicide car bombings. Britain is on high alert for attacks after Madrid bombings that killed 202 people.

"There will be questions being asked about how they managed to get over the fence and up the building," a Scotland Yard spokeswoman said.

"MAKE TEA, NOT WAR"

Thousands gathered in London for a peace march, one of many taking place across Asia, Europe and the United States against the U.S.-led war backed by Britain that toppled president Saddam Hussein, and an occupation marked by guerrilla resistance.

In London, protesters carried "Wanted" posters bearing the faces of Blair and U.S. President George W. Bush. Banners declared "Make tea, not war".

People dressed as weapons inspectors carried an inflatable nuclear missile to highlight the failure to find weapons of mass destruction in Iraq and Britain's retention of a nuclear deterrent.

Hundreds of black balloons were due to be released in memory of those who died in Iraq and in Madrid.

"We don't want any more lies or wars," said Ruth Tanner, a spokeswoman for the march's joint organiser, the Campaign for Nuclear Disarmament (CND).

Numbers were well down on a pre-war demonstration in London last year which attracted hundreds of thousands.