Here, they give some of their reasons for protesting:
I hope that President Bush, who is acting like a cowboy, will recognise that an era of western films is over.
Tomoharu Yamauchi, Tokyo
We believe that the vast majority of people in the United States don't want a war, they want money spent on education and human needs and not weapons of mass destruction.
Tony Murphy, campaigner, Washington
There's been too much capital invested in this war for it not to happen - but we're making our position clear. We're saying 'no'.
Adam Conway, Shannon, Ireland
Sign your name on a suicide attack on US interests, so we can fight an American attack along with Iraq.
Crowds in Beirut
The American is a coloniser who has come to cause destruction.
Protesters in Bahrain, home to the US navy's Fifth Fleet and thousands of US military personnel
I'm not really convinced that half of the American people want to invade Iraq. I think there is an eagerness to be patriotic... but there is confusion as to why.
Richard Foster, Maryland
Mara Verheyden-Hilliard, US protest organiser
You cannot take a nation to war unless it is united, and there is massive opposition in Britain.
Tony Benn, veteran UK politician, London
"We want to express our opposition to a war on Iraq. We want to join those who are demonstrating today all over the world.
Suha Bayumi, Cairo
Iraq isn't your ranch, Mr Bush.
Moscow banner
Don't impose war on Iraq.
Farrukh Sohail Goindi, Lahore