Union leaders, MPs and anti-war activists will address a rally in London's Trafalgar Square following a march. The protest, to mark the third anniversary of the start of the conflict, is being organised by the Stop The War Coalition, CND and the Muslim Association of Britain.
Demonstrations against the continued presence of US and British troops in Iraq will be held in 200 cities across the world, including Basra, Baghdad, New York, Madrid, Rome, Sydney, Tokyo, Toronto and Dublin.
In Sydney, protesters dressed up as George Bush carrying the Grim Reaper's scythe and chanting "End the war now!" and "Troops out!".
Lindsey German of the Stop the War Coalition said: "A solution to the chaos caused by illegal war in Iraq will only be possible when the occupying foreign armies have all been removed."