Veteran anti-war campaigner Brian Haw has died, a message on his website said.
A statement on brianhaw.tv said that "our father" Mr Haw had lost his battle with lung cancer yesterday.
The message, addressed to friends and supporters, said: "It is with deepest regret that I inform you that our father, Brian, passed away this morning.
"As you know he was battling lung cancer, and was having treatment in Germany.
"He left us in his sleep and in no pain, after a long, hard fight."
Protester Haw had been battling for his right to remain living in a tent in Parliament Square.
The statement, signed "Brian's family" continued: "With your help we have been able to share months more than we should have had with him, and for that we are eternally grateful.
"We would like to have this time to be together as a family, to share in the love he gave us, and respectfully ask that you allow us this time undisturbed.
"We will make further arrangements known to you all in due course."
Fellow members of the Parliament Square Peace Campaign said the authorities "should forever be ashamed of their disgraceful behaviour towards Brian".
He had been stationed in Parliament Square for ten years, and had fought of a series of legal objections to his presence there.
The latest saw the Greater London Authority get him and his supporters thrown off the grass area at the centre of the square.
Later this year Westminster Council is set for a court bid to get the camp moved off the pavement, which could see it removed permanently.
Mr Haw, 62, began his round-the-clock protest opposite the Houses of Parliament against the UK's policy in Iraq and elsewhere on June 2, 2001.
It began as an angry response to economic sanctions and British and American bombing raids on Iraq, but the scope grew wider after the 9/11 attacks on New York and Washington DC, and the invasions of Afghanistan and Iraq that followed.
His tent and ragtag collection of horrific pictures of war victims and hand-written posters with slogans like "baby killers" was a familiar sight in the square.
Civil rights campaigners got behind the protester as he saw off various attempts to force him to move.
In November 2004, ministers announced provisions in the Serious Organised Crime and Police Bill that could have seen him removed from Parliament Square.
The legislation, passed in April 2005, restricted the right to protest in designated areas within 1km (about half-a-mile) of the Houses of Parliament.
Initially, the High Court ruled Mr Haw's protest was not covered by the Act because it started before the new law came into effect.
But the case was taken to the Court of Appeal which, in May 2006, ruled he would have to apply to the police under the Act for permission to continue his demonstration.