Tens of thousands of anti-war protesters have marched peacefully on the White House to express opposition to a possible US attack on Iraq, some chanting slogans accusing President George W. Bush of planning genocide.
Thousands more people took part in anti-war demonstrations in San Francisco, Berlin, Amsterdam and other cities.
"This is going to be an ugly, unnecessary fight. Most of the world is saying 'no' to it," civil rights leader the Rev. Jesse Jackson told the crowd at the Vietnam Veterans Memorial in Washington. "Pre-emptive, one-bullet diplomacy, we cannot resort to that."
In Washington, actress Ms Susan Sarandon, who supports numerous liberal causes, accused Mr Bush of having "hijacked our losses and our fears." Sarandon said terrorism could not be fought with violence and that most Americans did not want a conflict.
"Let us resist this war," Ms Sarandon told the cheering crowd. "Let us hate war in all its forms, whether the weapon used is a missile or an airplane."
Demonstrators of all ages, many religions and many nationalities gathered at the Vietnam Veterans Memorial before marching behind Jackson to the White House. Mr Bush, however, was in Mexico for a summit of Pacific Rim leaders.
Police did not give an official estimate of the size of the crowd in Washington. Tony Murphy, an organiser of the event, said 150,000 people participated. Other observers put the figure between 40,000 and 50,000.
In San Francisco, known for its liberal politics and history of activism, a crowd that police estimated at about 42,000 marched near the city's historic Ferry Building to its Civic Centre.
In Germany, demonstrations were staged in about 70 towns and cities. The largest was in Berlin, where almost 10,000 people marched. In Amsterdam, some 4,000 people rallied in heavy rain to protest against US policy.