The AP has a story quoting Bill Dobbs from United for Peace and Justice about what happpened at the end of the march.

The organizers of an antiwar march said Sunday police escalated tensions by trying to clear a downtown park too quickly, while Mayor Bloomberg blamed a handful of unruly protesters for the injuries of 17 police officers.

“Most people behaved themselves, said their peace and went home,” Bloomberg said, a day after thousands marched through the city to show their opposition to the war in Iraq. “Unfortunately, a handful of people really got out of control and injured 17 New York City police officers who were there to protect us all.”

...

United for Peace and Justice spokesman Bill Dobbs said four medics volunteering for the group were pepper-sprayed by police at Washington Square Park, while others reported seeing protesters handcuffed too tightly.

Dobbs said police were cooperative and helpful until the protest reached the park. He said officers then tried to clear the area of thousands of people too quickly, and raised tensions by massing in riot gear and on horseback.

“It was premature to clear those streets,” Dobbs said. Once officers massed in riot gear, “the police became lightning rods. It’s regrettable.”

Several protesters also complained that they were shoved by officers and sprayed with Mace, Dobbs said.

“Whatever happened down there ought to have been defused with some common sense and not trying to clear streets prematurely,” he said.

Police officials had no immediate comment Sunday. Officials had said that the protesters’ march permit expired at 4 p.m. Officers began ordering people to disperse after the permit expired.

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Interesting --The NY Post article has this:

The peaceful mood turned truculent when cops started clearing the park around 3:30 p.m. - about a half-hour before the march was expected to end.

Unless if was dangerously crowded in Washington Square Park itself, it is NOT illegal for people to be in the park with anti-war signs. It is a public space, and one's presence there should not be at the whim of the NYPD.