Anti-war protester Brian Haw, who won a court battle to maintain his Parliament Square vigil which began in 2001, has been arrested.
Mr Haw's vigil began because he disagreed with sanctions against Iraq
The 56-year old father-of-seven, from Redditch, Worcestershire, was with a fellow campaigner when the arrest occurred, supporters said.
Mr Haw was arrested on suspicion of obstructing police.
Both Mr Haw and the other campaigner, Barbara Tucker, were taken into custody but were later released.
A Scotland Yard spokeswoman said: "A man and a woman were arrested at 2pm in Parliament Square."
The woman was arrested on suspicion of unlawfully demonstrating in Parliament Square, and she allegedly refused to give her details to police when requested.
Scotland Yard said the pair had been released pending a decision by the Crown Prosecution Service.
Last July a High Court ruled Mr Haw could continue his protest, which began initially over the international sanctions regime imposed against Saddam Hussein's Iraq.
New legislation means that police permission is required before mounting a demonstration within a mile of parliament.
But the court ruled that the requirement did not apply to Mr Haw, as his protest began in June 2001, which was before the law came into force.
Mr Haw's dedication is well known and he did not even return home last Christmas.
Speaking at that time he said: "We're killing each other - dropping bombs on our children. Someone somewhere has to do something to stop this madness."