The start of war against Iraq has drawn a barrage of criticism from leaders around the world and brought thousands of demonstrators onto the streets.
Russian President Vladimir Putin said the US-led military action was completely unjustified, while China said the strike violated the United Nations charter.
President Jacques Chirac of France expressed regret at the launch of hostilities without UN backing.
Support for the action came from, among others, the leaders of Australia, Japan, the Philippines and South Korea.
Around 100,000 people, including many school students, marched to the US embassy in the Greek capital, Athens, to demand a halt to hostilities.
Their calls were echoed in Cairo where Egyptian demonstrators tried to reach the American embassy to express Arab anger at the war.
A few hours earlier, widespread anti-war demonstrations took place across Australia and in Indonesia, the world's most populous Muslim nation.