Thousands of anti-war protesters are marching in Washington to demand the withdrawal of US troops from Iraq.
Organisers hoped for a turn-out of around 100,000
Organisers said they were expecting 100,000 to turn out for the 11-hour rally, march and concert near the White House and Washington Monument.
Opinion polls show a majority in the US believe the war in Iraq is going badly and US troops should be brought home.
A few hundred supporters of President Bush's policy in Iraq also gathered in Washington for a counter protest.
Anti-war rallies were also being held in other cities across the US as well as in London, Paris and Rome but in many cases the numbers were down on protests in previous years.
Demonstrators travelled from far and wide for what organisers said was the largest rally in Washington since the start of the war.
They carried banners reading "Bring the Troops Home Now" and "Bush Lied, Thousands Died."
"We have to get involved," said Erika McCroskey, 27, who travelled from Des Moines in Iowa with her mother and sister, to take part in her first demonstration.
Another demonstrator, 60-year-old Paul Rutherford, said he was a Republican who still supported President Bush, except over 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 was quoted by the Associated Press as saying.
Among those joining the protest was Cindy Sheehan, the mother of a fallen soldier in Iraq, whose protest outside President Bush's Texas ranch during the summer mobilised many anti-war supporters.
"We need a people's movement to end this war," she told the crowd. "We'll be the checks and balances on this out-of-control criminal government."
London was among a number of cities worldwide holding marches
President Bush appealed to Americans in a radio address last month to be patient.
"Our efforts in Iraq and the broader Middle East will require more time, more sacrifice and continued resolve," he said.
Meanwhile, anti-war rallies were also being held in Los Angeles, San Francisco, Seattle and in major cities across Europe.
British police said around 10,000 people took to the streets in London, although organisers put the figure at nearer 100,000.
A small rally was held in Paris, and in Rome dozens of demonstrators held up banners and peace flags outside the US Embassy.