Brian Becker, the national coordinator of the Answer Coalition and a member of the Party of Socialism and Liberation, said the group held out little hope of influencing either the president or Congress. “It is about radicalizing people,” Mr. Becker said in an interview. “You hook into a movement that exists — in this case the antiwar movement — and channel people who care about that movement and bring them into political life, the life of political activism.”

In a speech before the march, Cindy Sheehan, who made headlines in 2005 camping outside the Mr. Bush’s Texas ranch after her son was killed in Iraq, called the president and his military advisers “war criminals.”

“We want the people in the White House out of our house and arrested for crimes against humanity,” Ms. Sheehan said.

As they gathered before the march, the protesters met what several veterans of the antiwar movement described as an unusually large contingent of several hundred counterdemonstrators. Many were veterans in biker jackets who said they had come to protect the nearby Vietnam Memorial, citing rumors that had circulated among veterans groups that the demonstrators planned to deface it.

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Crossing the bridge toward the Pentagon, the marchers met another group of about 50 counterdemonstrators by the Arlington Cemetery, one holding a sign that said: “Go to hell traitors. You dishonor our dead on hallowed ground.”

Near the Pentagon, police officers in riot gear spread across the road, effectively blocking the demonstrators from approaching the building. Five people were arrested by the Pentagon Force Protection Agency for “failure to obey a lawful order,” said Cheryl Irwin, a Pentagon spokeswoman.

Many in the crowd said they were unfamiliar with the Answer Coalition and puzzled by the many signs about socialism. Several said they had come from across the country for a chance to voice their dismay at the war.

Alan Rainey, an adjunct professor and small publisher from West Lafayette, Ind., said he had not attended a protest since 1973, not long after he had returned from military duty in Vietnam. On Saturday, he carried a sign with green clover and a St. Patrick’s Day theme. “Help drive the snakes out of the White House,” it said, depicting snakes with the faces of Mr. Bush and Vice President Dick Cheney.

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“This war is criminal,” Mr. Rainey said. “We impeached Clinton for a little indiscretion with an adult.”

Judy Creville, who came from Michigan, said she had opposed the war from the start but never attended a protest before. “They got on my last nerve,” Ms. Creville said. She came with two sisters from Michigan and Iowa, and all three wore pictures of their school-age grandchildren.

Zohrea Whitaker said she came from Sacramento for the protest. “I have a son serving over there, and I want him home,” Ms. Whitaker said.