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.
Anti-war protesters took to Sydney's 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.
Mr Putin urged the US to halt what he called the unjustifiable attack on Iraq - an attack which questioned a basic principle of world order.
"If we install the rule of force in place of international
security structures, no country in the world will feel secure," Mr Putin said.
Mr Chirac, who had voiced opposition to any UN resolution that automatically led to military action, said: "I only hope these operations are as fast as possible, with the least fatalities, and that they do not lead to a humanitarian catastrophe."
Chinese Foreign Ministry spokesman Kong Quan said the military operation violated the principles of international law.
"They ignored the opposition of most countries and peoples of the world and went around the UN Security Council to being military action against Iraq," he added.
The Vatican said it was "deeply pained" by the conflict and deplored the interruption of peace efforts.
However, there were some words of support for Washington.
"I understand the start of the use of force by America and support it," Japanese Prime Minister Junichiro Koizumi said in a televised address.
"If a dangerous dictator possesses dangerous weapons of mass destruction, we will face a big danger. How to get rid of this threat concerns people around the world," Mr Koizumi said.
Philippine President Gloria Arroyo said Manila gave both political and moral support for the action against Iraq.
South Korean President Roh Moo-Hyun also pledged his government's support for the US-led war.
But President Megawati Sukarnoputri of Indonesia voiced her disapproval.
Washington had pushed the UN to one side to wage war, she said.
So far there has been little official comment from the Middle East, but Iranian Foreign Minister Kamal Kharazi condemned what he said was the "illegitimate and unjustifiable attack" on neighbouring Iraq.
The Secretary-General of the Arab League, Amr Moussa, said it was a sad day for all Arabs.
There has also been anger and dismay around South Asia.
Pakistan said it deplored Thursday morning's attack, while India said the attacks were unjustified.