Dozens Arrested During Brooklyn Bridge Protest Against Police Violence

Dozens of participants in a march intended to protest police violence were arrested on Tuesday as the demonstration crossed the Brooklyn Bridge and caused major delays there during the evening rush.

Stephen Davis, the Police Department’s chief spokesman, said 34 people had been arrested by 6:40 p.m. That number was expected to rise overnight as precincts in Brooklyn and Manhattan reported arrests, he said.

There were also reports of assaults on police officers, including one involving an off-duty police sergeant who was punched in the face when he got out of his car, Mr. Davis said. The officer was treated at Lower Manhattan Hospital, and the assailant has not been caught, Mr. Davis said.

The police said they were also investigating a report of an officer being struck in the head by a beer bottle.

In a statement, Mayor Bill de Blasio said the violence against the police officers was unacceptable.

“And any other person who might use the right to peaceful protest as cover to initiate violence, cause mayhem or incite disorder — whether against the police, the people or property of our great city — should consider themselves on notice that New York City will not stand for it.”

Witnesses and accounts on social media described protesters being rounded up and put in police vans. There were also reports of objects being thrown at police officers and scuffles between officers and demonstrators.

The Stop Mass Incarceration Network organized the march, one of several across the country. About 400 people participated in the early part of the demonstration, which began at Union Square at 2 p.m. and wound down Broadway toward Police Department headquarters in Lower Manhattan.

Around 4:15 p.m., some of the protesters split off and went to the Brooklyn Bridge, where they broke through a police barricade. Some jumped over a fence and onto the westbound traffic lanes.

Traffic in those lanes was disrupted for several hours.

Some protesters continued on Flatbush Avenue toward the Barclays Center.

The demonstration follows the most recent in a string of fatal police encounters with unarmed black men.

Walter Scott, 50, of North Charleston, S.C., was shot and killed by a police officer on April 4. A cellphone video showed the officer, Michael Slager, firing eight shots as Mr. Scott ran away, and later dropping his service Taser next to the body. Mr. Slager was fired and was charged with murder.

Before the march, Cornel West, a prominent black scholar, spoke at Union Square about what he said was his frustration with black leaders’ inability to produce charges and convictions against the officers involved in the shootings.

“Don’t be confused by some black faces in high places,” he said. “For seven years, there have been black and brown brothers and sisters shot down by the police.”

He noted that the president, the attorney general and the secretary of homeland security were all black. “And not one policeman sent to jail,” Mr. West said.