A mile-long procession of demonstrators marched through downtown San Francisco on Saturday to demand President George Bush be ousted from office and US soldiers brought home from Iraq.
Protest organisers from the group Act Now to Stop War and End Racism (Answer) estimated the crowd peaked at 250,000 people, while police said attendance was probably closer to half that number.
Opponents of the US-led war also marched in central London. Police said 10,000 people converged on Hyde Park but organisers said 100,000 had showed up.
In Washington, as many as 100,000 people protested. But Bush was out of town, monitoring hurricane recovery efforts from Colorado and Texas.
"We have to get involved," said Erika McCroskey, 27, who came from Des Moines, Iowa, with her younger sister and mother for her first demonstration, travelling in one of the buses that poured into the capital from far-flung places.
"Bush lied, thousands died," said one sign. "End the occupation," said another.
While united against the war, political beliefs varied in the Washington crowd. Paul Rutherford, 60, said he is a Republican who supported Bush in the last election and still does - except for the war.
"President Bush needs to admit he made a mistake in the war and bring the troops home, and let's move on," he said.
"We found that there were none and yet we still stay there and innocent people are dying daily," she said. Arthur Pollock, 47, said he was against the war from the beginning. He wants the soldiers out, but not all at once.
Some activists said public opinion
had tipped against the war
"We believe we are at a tipping point whereby the anti-war sentiment has now become the majority sentiment," said Brian Becker, national coordinator for Answer, one of the main anti-war organisers.
Cindy Sheehan, the California mother who drew thousands of demonstrators to her 26-day vigil outside Bush's Texas ranch last month, joined the protest.
Her 24-year-old son, Casey, was killed in an ambush in Sadr City, Iraq, last year.
Small rallies were also held in Copenhagen, Damascus, Helsinki, Paris, Rome and Seoul. Others were scheduled in Los Angeles and Seattle.
In London, thousands of protesters marched to demand that Britain pull its troops out of Iraq and to send a strong message to Prime Minister Tony Blair's Labour Party as members headed to their annual conference.
The UK march coincided with the
Labour Party's annual conference
Clashes between fighters and British troops in the southern Iraq town of Basra this week highlighted the urgent need to withdraw, said the Stop The War Coalition, which organised the march.
"Enough is enough. It is now time, once again, for the British people to step forward into the streets and insist that, this time, we will not be ignored," coalition official Lindsey German said.
Anger at Blair
As the march passed the entrance to Downing Street, Blair's home, some protesters shouted "down with Downing Street" and "stop the bombings".
"The British people are increasingly realising that they have been told more and more lies about the war," said protester Lance Corporal George Solomou, a British soldier who refused to go to Iraq.
A spokeswoman for the Metropolitan Police said an estimated 10,000 people had joined the march.
"I am totally overwhelmed," said Peter Brierley, whose son Shaun, 28, died serving in Kuwait in 2003.
"Looking at what has happened in Iraq through this last week, it is obvious that Iraq does not want troops there. If they do not bring them out, there will be more families like us."