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PORTLAND, Ore. - Marking the fourth anniversary of the invasion of Iraq, thousands of demonstrators crowded into a grassy stretch of downtown Portland on Sunday to call for an end to the war.

Raed Jarrar, an Iraqi-Palestinian writer, said the war has become one of the occupiers versus almost everyone else in the country, with only a small number of Iraqis collaborating with U.S. forces.

The war "is happening because of the occupation, not in spite of it," Jarrar said.

"There are Iraqis who can rebuild their country," he said. "They don't need someone to come from thousands of miles away to tell them how to treat their neighbor. They are the only ones who can end this violence."

At the end of the march, a small group of protesters left the route and continued walking. Police on bicycles followed them, and then a scuffle began, with police using pepper spray.

At least two people were detained.

The Portland protest was one of several around the country over the weekend.

The crowd numbered in the thousands. Organizers said there might be as many as 10,000 people lined up for a 24-block march beginning and ending at Portland's South Park Blocks.

In San Francisco, thousands of marchers closed down a major downtown thoroughfare in a largely peaceful display of opposition to the Iraq war.

Although some marchers said they had been protesting the war for years, others said they showed up on a sunny afternoon to rally against the war for the first time.

Dozens of police on foot and motorcycle blocked traffic and kept an eye on the crowd, which stretched for blocks through the city's financial district. No arrests were reported by mid-afternoon.

A smattering of counter-protesters waving American flags also gathered in what they described as a show of support for U.S. troops.

In New York, hundreds of peace protesters converged on a park near the United Nations. They called for the end of funding for the war.