A man who scaled the scaffolding on the Houses of Parliament has been arrested.
The protester climbed about 66ft (20m) up on to the scaffolding facing Bridge Street using a ladder and unfurled a rainbow peace flag.
He displayed another banner reading: "We respect the soldiers. We do not support the war".
The man was arrested on suspicion of trespass, a Metroplitan Police spokesman said.
A group of protesters, who attempted to enter the Parliament complex, were dressed in yellow workmen jackets and white hard hats.
All the protesters live in the "peace camp" in Parliament Square.
High Court action
A Metropolitan Police spokeswoman said the group were spotted at 0900 BST.
She said: "One man has managed to get through and climb the scaffolding. One man was detained at the perimeter wall and he has been arrested and taken to a central London police station.
"A third man managed to get to the foot of the scaffolding where we arrested him and took him to a central London police station."
The camp, a collection of tents dubbed Democracy Village, sprang up on 1 May and anti-war demonstrators, climate change activists, communists and anarchists have been living there.
On Tuesday police searched the camp ahead of the State Opening of Parliament.
Anti-war campaigner Brian Haw, who has been living on the green since 2001, and another protester Barbara Tucker were charged with obstructing police following their arrest during the search.
Mayor Boris Johnson has also announced that he will approach the High Court to take action on the tent-city on Parliament Square.
The green is owned by Greater London Authority.