Thousands plan to rally in Washington this weekend in the capital's first mass protest of a potential war in Iraq.

Saturday's protest is set to coincide with marches in San Francisco and abroad, including events in London and Tokyo. The rally's sponsors predict that, in all, hundreds of thousands will participate.

All of the rallies are being planned by the same coalition of antiwar and anti-racism groups that organized a pro-Palestinian march here in April. Police estimated that that event drew 75,000 participants.

Organizers of Saturday's protest say that more than 250 buses are bound for Washington. About half will come from college campuses, the time-tested spawning pools for protest movements. But a significant fraction of the rest, organizers said, are coming from the mosques and Muslim associations that rallied with the coalition under the Palestinian flag in the spring.

"I think we broke new ground in April, being able to forge alliances with the grass-roots Muslim community," said Brian Becker, co-director of the International Action Center, one of the groups that make up International ANSWER, the coalition that is organizing the protest. "Trust and a strong bond was established there, and that has carried over."

The demonstration is scheduled to start at 11 a.m. Saturday at Constitution Gardens, adjacent to the Vietnam Veterans Memorial near Constitution Avenue and 21st Street NW. The brief program will feature several speakers -- Jesse L. Jackson, Al Sharpton, former attorney general Ramsey Clark and others, including singer Patti Smith. It will be followed by a march to the White House.

Saturday's march is not intended to disrupt the city or to create traffic problems, Becker said. Organizers are promoting it as being dissimilar to last month's anti-capitalist demonstrations against the International Monetary Fund and the World Bank, though some groups that participated in the IMF and World Bank rallies said last week that they plan a "feeder march."

Activists from groups including the Anti-Capitalist Convergence, the D.C. Anti-War Network, the D.C. Statehood Green Party and Left Turn plan to gather at the Columbia Heights Metro stop and march past residential neighborhoods and the White House before joining the larger group at Constitution Gardens.

Adam Eidinger, an organizer with the D.C. Statehood Green Party, said the aim is to get attention. Some protesters are planning to take off their shirts to protest President Bush's taking "the shirts off our backs" to pay for the war, he said.

A counter-demonstration is planned by the D.C. chapter of Free Republic, the national conservative group, from 11 a.m. to 3 p.m. at 17th Street and Constitution Avenue NW.

D.C. Police Chief Charles A. Ramsey said extra officers -- including platoons from the civil disturbance unit -- will be on the street. But he said that the peacefulness of April's rally leads him to believe that Saturday's will also go smoothly.

"They're always very peaceful; you can work with them," Ramsey said of International ANSWER demonstrators. "They just want to come in and protest. They don't want to cause any damage."

That reputation has helped organizers attract large numbers from the Muslim community, according to Mahdi Bray, executive director of the Muslim American Society Freedom Foundation. Because many immigrated from countries where protests are not allowed, Bray said, they were initially reluctant to embrace activism in the United States.

"For a lot of them, the Palestine rally was the first demonstration they had ever seen," Bray said. "They were concerned -- because they are law-abiding American citizens and patriots -- and they didn't want to be seen as troublemakers. But now they've seen they can do that, and that they're not alone."

Bray said protesters believe that the Bush administration has manufactured a rush to war without sufficient cause and that the nation's foreign relations strategy is being driven by imperialistic impulses that infringe on the rights of other people. They said that a war on Iraq would hurt the Iraqi people more than it would hurt the Iraqi president.

"Basically, you'd be punishing the wrong folks," Bray said. "Saddam Hussein is a dictator -- we all understand that. We have empathy for the people of Iraq, not the regime."

Bush's signing of a resolution last week authorizing the United States to attack Iraq energized the protest movement, according to those who are helping to arrange bus trips to Washington.

In Chicago, Bill Massey, an antiwar activist, said he now has enough people signed up to fill six buses. Two others have been reserved by a local mosque and a Palestinian advocacy group.

"The majority of people coming are actually not from the Muslim or Palestinian [communities], although there is a lot of participation there," Massey said. "Most come from all walks of life. [It's] a whole new layer of people coming from churches, from community groups, from everywhere. The African American community, in particular, is really coming out against this."

At Howard University, Peta Lindsay, a freshman, has been passing out fliers and spreading the word about the march for weeks.

"The response has been overwhelming," said Lindsay, who is helping International ANSWER coordinate student participation on college campuses. "And it's not just this October 26 march. There have been other, ongoing campaigns against the war. But everyone acknowledges that October 26 will be the mass convergence."

Smaller demonstrations against a potential war with Iraq have become common in recent weeks across the country. One of the largest was Oct. 6 in New York, when a crowd of 10,000 to 20,000 rallied in Central Park. No U.S. rally, however, has matched the size of the antiwar demonstration in London on Sept. 28, which drew an estimated 200,000. The British turnout has become one that some of those coming to Washington hope to match.

"We can show in one space how big this movement is," said Matthew L. Schwartz, a freshman at the University of Buffalo who has helped charter two buses to Washington. "When you flood the streets in masses, you can't ignore the message. You have to see that people don't want this war."

Staff writers David A. Fahrenthold and Manny Fernandez contributed to this report.