New York ? A march against the Bush administration turned ugly Monday when a protester repeatedly stomped and punched a plainclothes detective and hundreds of officers in riot gear pushed demonstrators away from the site of the Republican convention, witnesses and police said.

The detective, William Sample, was briefly knocked unconscious and was hospitalized with head injuries that were not life-threatening. His assailant escaped and was being sought by police.

Police Commissioner Raymond Kelly called it “a blatant, vicious attack.”

The unrest occurred a day after tens of thousands of protesters took to the streets of Manhattan for a march that ended up being largely peaceful despite fears of violent clashes with police. It was the largest convention-related protest in U.S. history.

Monday’s protest began when a crowd of several thousand outside U.N. headquarters negotiated with police to march to Madison Square Garden despite not having a permit. Police proposed a route to a permitted protest area, and demonstration leaders accepted.

“They asked if they could march, and we said yes,” police Assistant Deputy Commissioner Tom Doepfner said. “We try to be nice.”

What resulted was an astonishing sight: Marchers trooped down city streets with no barricades, passing parked cars and inadvertently setting off some alarms as police — some on scooters — steered the restless, unwieldy crowd through midtown Manhattan rush-hour traffic. Cars were backed up 10 blocks in places. The injured detective was knocked off his scooter amid the melee.

As the march came within two blocks of the Garden, police officers began setting up barricades and clashed with protesters who tried to break through them. Hundreds of police in riot gear and on horses swept in to disperse the crowd, shouting, “Move!” At least 10 arrests were made as protesters yelled back, “Whose streets? Our streets!”

New York City police officers restrain a demonstrator during the March

The crowd sang protest songs, heard speakers read poetry and waved signs. Among them: “Billions for the war, still nothing for the poor.” One protester chanted “Racist, sexists, anti-gay, R-N-C go away.”

The march was organized by a group called the Poor People’s Economic Human Rights Campaign.