Environmental campaign group Greenpeace said they organised the stunt to protest against the British and United States governments' policies over Iraq.

"We want to send a clear message to Tony Blair that we and the British people are fed up with the half-truths and evasions on Iraq," Greenpeace executive director Stephen Tindale told Reuters.

He said the pair were buffeted by strong winds during their one-hour climb after clambering over two security fences at the base, evading some of the tightest security measures in Britain.

Armed police guard parliament and concrete blocks ring the building to prevent suicide car bombings. Britain is on alert for terror 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. "But at the moment we are trying to focus on the operational side of things."

A Greenpeace statement said the climbers would stay until nightfall on the tower which houses the clock's Big Ben bell.

Hundreds of thousands were expected to take part in demonstrations across Asia, Europe and the United States against the war that toppled president Saddam Hussein and an occupation that has been marked by guerrilla resistance.