Activists urging an end to Britain’s role in overseas’ wars gathered in Trafalgar Square, central London, for a march on parliament.
The protest, organised by the Stop the War Coalition, was called to mark the fifth anniversary of the invasion of Iraq. Demonstrators called for no US attack on Iran and for an end to the “siege” of Gaza.
Organisers in London estimated up to 40,000 people joined the protest. Police said no arrests were made. They estimated numbers at 10,000.
The London protest was one of a number taking place in countries around the world on Saturday, organisers said. Protesters heard speeches from the leaders of a range of groups including CND, and the Palestinian Solidarity Campaign.
Former Labour cabinet minister Tony Benn said: “The troops in Iraq have caused devastation. It’s the same in Afghanistan.”
Green MEP Caroline Lucas called for the Prime Minister and his predecessor to be prosecuted for war crimes. She said: “Tony Blair and Gordon Brown should be tried for war crimes at the international court in the Hague.
“They need to know you cannot bomb your way to peace.”
Former CND leader Bruce Kent described the protest as an “orchestra of peace”.
“I think the common agreement is for a peaceful and just world. I am speaking about nuclear weapons but the war in Iraq is part of the same campaign.”