Anti-war demonstrators are gearing up for the second consecutive weekend for a major street protest in Washington on Saturday in a key test of whether sentiment is growing against the war in the Middle East.

The National Campaign for Peace in the Middle East, a coalition of student, religious, labor and human-rights groups, has told police to expect 50,000 to 75,000 protesters. They are scheduled to rally at noon on the Mall at Third Street NW near the Capitol, then march down Pennsylvania Avenue NW to the Ellipse just south of the White House for a second rally.

Scheduled speakers include Jesse L. Jackson; Molly Yard, president of the National Organization for Women; and various student, church and union representatives.

Organizers said some groups are planning traffic blockades and other acts of civil disobedience near the White House. Since Jan. 1, about 150 protesters have been arrested in Washington, most for demonstrating without a permit.

A group called the National African-American Network Against U.S. Intervention in the Gulf intends to assemble at Meridian Hill Park, also known as Malcolm X Park, then march to 14th Street and Pennsylvania Avenue NW to join the main group.

Organizers said they hope the days since last Saturday's rally in Lafayette Square have given protesters more time to gather strength, and that fading expectations of a quick war will galvanize additional anti-war sentiment. Surveys indicate continued widespread backing of President Bush and U.S. military action against Iraq.

"We have at least 500 buses confirmed from out of town," said campaign logistics coordinator Phillis Engelbert, "and that's not counting New York," traditionally a big contributor to rallies.

Police said they are bracing for at least as many demonstrators as on Jan. 19 -- a raucous, often angry crowd estimated by police to number 25,000 and by organizers to number 100,000. That rally, sponsored by the National Coalition to Stop U.S. Intervention in the Middle East, came three days after the U.S. attack on Iraq.

Like the Campaign for Peace, the Coalition to Stop U.S. Intervention has demanded an immediate end to the war, U.S. withdrawal and diversion of military funds to domestic needs, such as housing and social services.

But unlike the campaign, the coalition has not condemned Iraqi leader Saddam Hussein for his invasion of Kuwait. The coalition also has called for an end to economic sanctions against Iraq. The campaign said it has taken no position on sanctions, except to say there should be no embargo on food and medicine.

The campaign has tended to draw more religious, pacifist and student groups, and the coalition has tended to attract tougher-talking advocates of radical causes, including revolutionary socialism and Palestinian nationalism. But the two organizations overlap philosophically in some ways, and periodically issue statements of mutual support.

"Basically, both groups want to stop the war," said Engelbert, of the campaign. "I think a lot of the people who came here for the Jan. 19 rally will come to ours, as well."

Will the anti-war movement grow as the Mideast conflict lengthens? "It could go either way," said Lisa Fithian, a campaign coordinator. "Some people will still be supportive of the war . . . . But there's also a lot of despair."

Asked if there had been an erosion of support by Jewish organizations or indviduals since the Iraqi missile strikes against Israel, Fithian said, "I haven't seen a drop-off yet, but there's been a lot of debate within the Jewish community."