A rally in the shadows of Washington's political and military institutions anchored dozens of smaller protests throughout Asia, Europe, the Middle East and the United States.
In Washington, police said 30,000 marched through the streets, part of a much larger crowd that packed the east end of the National Mall and spilled onto the Capitol grounds.
"We stand here today, a new generation of anti-war activists," Peta Lindsay from International Answer, the main organizers, exhorted the spirited masses in a biting cold. "This is just beginning. We will stop this war."
On-scene, CBS News Correspondent Joie Chen said the protesters came from all walks of life: "Young, old, veterans, veteran activists; different values, different views, but united in an effort to stop a war before it starts.
Police reported few arrests in the rally, which preceded the march past Marine barracks to the Washington Navy Yard.
"We don't want this war and we don't want a government that wants this war," said Brenda Stokely, a New York City labor activist. A sign branded America, not Iraq, a "Rogue Nation." Another said, "Disarm Bush."
Activists invoked the nonviolent legacy of Martin Luther King Jr. on the long weekend that marks the civil rights leader's birthday, and booed President Bush, who was at Camp David, Md.
King's historic "I have a dream" speech rang out from the opposite end of the mall, the Abraham Lincoln Memorial, before a crowd of more than 200,000 in 1963.
"Mr. Bush hung Dr. King's picture up in the White House last year but he need to hang up Dr. King's words," the Rev. Al Sharpton, a Democratic presidential candidate, told the demonstration.
Added civil rights activist Jesse Jackson: "We march today to fight militarism, and racism, and sexism, and anti-Semitism, and Arab-bashing."
Terrence Gainer, chief of the U.S. Capitol Police, said "about 30,000 people moved out on the march route," a two-mile trek from the huge rally.
Mr. Bush believes that protesting "is a time-honored part of American tradition and it's a strength of our democracy," White House spokeswoman Jeanie Mamo said. But spokesman Ari Fleisher said the protesters don't represent the views of the majority of Americans.
Demonstrators hoped the protests and more ahead would win over an American public unsettled by the prospect of an Iraq war yet supportive of Mr. Bush's leadership. Some dared hope their activism would give his administration pause.
"Our voices ought to matter." said Joyce Townsend, 69, who came from Detroit on a bus with members of her church.
As with any big Washington rally, the main cause made room for other causes.
"Free Palestine" was one of them. Racism and genocide were others.
"The underlying motives for this government's actions have always been greed and racism," said Moonanum James of United American Indians of New England.
"In the spirit of Dr. King, in the spirit of Crazy Horse," he said, "no blood for oil."
Elsewhere, protesters denounced Mr. Bush's Iraqi policy in a major rally in San Francisco, where protesters came by the thousands.
"I'm hoping that the bus loads of people coming as far away as Oregon and Nevada give an indication that this isn't just the crazy loons in San Francisco - but we reflect the opinions of the entire United States," said Tim Kingston of the anti-war group Global Exchange.
At that rally, CBS News Correspondent John Blackstone says that while northern California is "famously liberal," and "demonstrations and dissent are nothing new here," the crows, in its size and intensity, reminded some veteran activists "of the glory days of the anti-war movement.
In Lansing, Mich., several hundred people met at a church before marching 20 blocks to the state Capitol. "It's just great enthusiasm here, and a great spirit of peacemaking," said the Rev. Fred Thelen from Cristo Rey Catholic Church.
In Des Moines, Iowa, about 125 protesters marched two miles in a bitter wind that made temperatures feel below zero. "Standing out in this kind of temperature is nothing compared to innocent people losing their lives in Iraq," said marcher Eric Kimmer, 32, a credit union worker.
About 400 people, many of them elderly, gathered in downtown Venice, Fla., to listen to anti-war speeches. "America cannot unsheathe the sword, and tell the rest of the world to brandish plowshares," said Methodist minister Charles McKenzie.
Demonstrators staged peace rallies worldwide, events that typically drew hundreds or fewer.
But 5,000 people marched through downtown Tokyo, carrying toy guns filled with flowers and wearing face masks that parodied Mr. Bush.
Larry Holmes, speaking for organizers of the Washington rally, said protesters everywhere sense war is close.
"It seems like it has a momentum and a sense of inevitability, and so we're rushing against the clock," he said. "So as they send the troops there and surround Iraq, we're sending the troops into the streets of Washington, D.C., so to speak."
Three dozen people stood by the Vietnam War Memorial to show support for Mr. Bush's policy and offer a contrary voice to the blitz of demonstrations.
"The protesters don't understand the threat" of Iraqi President Saddam Hussein, said Scott Johnson, 55, a Navy veteran from Minneapolis. "It's a war of liberation for people."
Overseas, police in the Netherlands detained 90 activists who tried to enter Volkel Air Force Base, where Dutch and U.S. forces are stationed, to conduct a "citizens' inspection of American nuclear arms."
Sixty protesters in Hong Kong shouted, "War, no," and in Pakistan, the familiar refrain "No blood for oil" rang out - a refrain that accuses America of wanting to attack Iraq only to control its oil wealth.
Several hundred people tried to march on the U.S. consulate in Lahore, but Pakistani authorities held the crowd back. Six were allowed to deliver an appeal to American officials to spare Iraqis from war.
Police in riot gear clashed with protesters in Cairo, Egypt.
At the 6,000-strong demonstration in Paris - the third nationwide anti-war demonstration since October - many shouted "Down with war!" and a few people set off firecrackers. Most French people oppose military intervention, polls show.
Near London, about 200 people demonstrated outside the barbed wire fence of a military base. Elsewhere in Europe, a protest in Goteborg, Sweden, gathered 5,000 peaceful demonstrators, while a few hundred people marched in the German cities of Cologne and Bonn.
More than 400 New Zealanders demonstrated in Christchurch. In Moscow, a few hundred people agitated outside the U.S. Embassy in a protest organized by a branch of the Communist Party. People turned their backs to the building, and signs called the United States a "Global Cannibal."
In the Syrian capital, Damascus, thousands marched with a message that was not all about peace. Many cried, "Our beloved Saddam, strike Tel Aviv," in celebration of Iraq's missile thrusts against Israel during the 1991 Gulf War and in hope Saddam would strike again.
In the Gaza Strip, Palestinians rallied under the same slogan.