A peace activist and his supporters are to mark 2,000 days of his continuous anti-war vigil in Parliament Square.
Mr Haw has fought off several eviction threats
Brian Haw, 57, set up camp opposite the House of Commons on 2 June 2001 to attack government policy on Iraq.
His anti-war placards and megaphone harangues have been a thorn in the side of ministers ever since.
Mr Haw, from Worcestershire, has resisted several attempts to have him silenced and removed, including a law actually devised with him in mind.
He will be joined by John McDonnell MP, Jenny Jones of the Greater London Authority's Green Party and representatives from civil liberties group Liberty.
Ms Jones said Mr Haw represents all those who oppose the war.
"His brave vigil in Parliament Square is a protest on behalf of the hundreds of thousands who know the war is unjust, illegal, and unwinnable," she said.
In July 2005, Mr Haw won a legal battle to continue his vigil due to a drafting error in a new law banning unauthorised protests in Westminster.
The Serious Organised Crime and Police Act 2005 states anyone wanting to demonstrate in a half-mile zone in central London must seek permission from the police.