ASHINGTON, Jan. 18 — In a show of dissent that organizers said "shattered the false myth of consensus," for a war with Iraq, tens of thousands of protesters representing a diverse coalition for peace converged here today for a rally and march against the Bush administration's threatened use of military force against Saddam Hussein's regime.
Energized by the British band Chumbawamba, which opened the Washington demonstration with a performance of a new antiwar song, a swelling crowd, packed densely to stave off the winter winds, filled several blocks west of the Capitol carrying signs, waving banners and chanting, "No war with Iraq."
Meanwhile, in San Francisco, swarms of demonstrators filed off buses, ferries and up from the subways along the waterfront for a march heading up Market Street and into the downtown. Among the protesters were a caravan of environmentalists in electric cars with signs that read "Go solar, not ballistic," and the Stroller Brigade, a group of Bay Area parents pushing their children through the crowds.
Both marches were sponsored by the activist group International Answer, after months of intense local organizing following a similar large demonstration in the capital last October.
In a show of solidarity with the march in Washington, which drew participants from around the country and was timed to coincide with the Martin Luther King holiday weekend, other antiwar activities took place nationwide. Thousands marched through downtown Portland, Ore., led by a drum ensemble and cheerleaders with multicolored pompoms. In Tampa, Fla., protesters rallied outside the gates of MacDill Air Force Base as military jets took off nearby. Other events were held overseas in cities including Tokyo, Paris, Cairo, and Moscow.
Many in the crowds seemed undeterred — even invigorated — by the steady and seemingly inexorable march toward a possible war, perhaps in a few weeks, as the United States and a few allies marshall troops, naval flotillas and air wings in a rapidly escalating mobilization in the Persian Gulf region. As protesters marched, Pentagon officials said that more aircraft carriers soon would be bolstering the numbers of attack aircraft in the region.
"The government is going to do what they are going to do regardless," said Mike Smith, 22, a student who was one of hundreds of people to arrive in Washington in a caravan of 11 buses from Chicago. "But at least by coming we can try to make sure that people in other countries know that all Americans are not down with this war."
Among the groups in attendance were the Gray Panthers, a social advocacy group; Code Pink, a women's group; Black Voices for Peace, an African-American group; and the Green Party, representing environmentalists.
"Local actions are critical, but there are times when it is necessary to amass an undeniable massive physical and vocal presence," said Mara Verheyden-Hilliard, a civil rights lawyer and spokeswoman for Answer.
Margaret Conway, 21, drove overnight with 10 friends from a theater group at the University of Michigan. Sager Williams, 51, a lawyer, came with friends from Annapolis, Md. Howard Marland, 60, a carpenter, came to the rally with a dozen people from Dumbarton United Methodist Church in Washington.
Protesters greeted one another and shared their backgrounds in small groups as a steady stream of speakers rallied the crowd for two hours from the stage. In addition to dozens of activists representing groups like the Muslim Student Association, Pastors for Peace and Global Exchange, there were several celebrity speakers.
Among them were the Rev. Jesse Jackson and the Rev. Al Sharpton, the actresses Jessica Lange and Tyne Daly, and Ron Kovic, the Vietnam veteran and antiwar activist. In San Francisco, the actor Martin Sheen and the singer Joan Baez participated in events.
"You all know what I do for work, this is what I do for a living," said Mr. Sheen, who plays the president on the television show "The West Wing." "If the people lead, the leaders will follow."
In addition to Answer, which drew support from 200 organizing centers around the country, the march from the National Mall to the Washington Navy Yard about two miles away benefited from the formation and growth over the past several months of other antiwar coalitions.
The two main groups, United for Peace, an umbrella group of more than 120 organizations, and Win Without War, a coalition of religious, business and civic leaders, have helped draw mainstream support by using patriotic antiwar messages. In recent weeks the groups have won high-profile backing from groups like labor unions, which have committed thousands of dollars and people to the organizing efforts.
Last week, a number of Republican business leaders lent their support by taking out a full-page advertisement in The Wall Street Journal opposing a war with Iraq.
Still, many of those marching today were not part of organized groups, but were simply skeptical and frustrated citizens who felt compelled to attend.
"The antiwar movement hadn't been very visible to us in our daily lives, and we thought we needed to stand up and be counted," said Vicki Rosenwald, 53, a research nurse from New York who attended the rally with her husband and a group of friends. "It's important for ordinary middle-aged, middle-class people to show up at these things because we can't be dismissed as campus radicals."
Two hours before the start of the antiwar rally here, supporters of the war effort held a counter protest on the National Mall, southeast of the Vietnam Memorial. Fewer than 100 people — mostly from two groups, one called Move-Out and another called Free Republic — waved flags as "The Star Spangled Banner" played over a portable speaker.
"We believe in America and what America stands for," said Joe Kernodle, a Vietnam veteran and spokesman for Move-Out. Many of the counter protesters were associated with the military, but Nina Burke and her husband, Steve, came from Fredericksburg, Va., as civilians. "We need to disarm Saddam before he sneaks a nuke into Chicago or New York, not after," Ms. Burke said.
