Paul Browne, a police department spokesman, said that the protesters initially stayed on the elevated pedestrian walkway in the center of the Brooklyn Bridge, which thousands of New Yorkers use every day.
"There were no problems until a group broke away and started chanting, 'Take the bridge, take the bridge,'" he said.
At that point, many protesters poured into the traffic lanes bound for Brooklyn, and police began arresting them for blocking cars, Browne said.
The demonstration was led by the group Occupy Wall Street, which has been camped out in a plaza in Manhattan's Financial District for nearly two weeks, staging various marches. On Saturday, it had organized an impromptu trek to Brooklyn. Protesters walked in thick rows on the sidewalk up to the bridge, at which point many veered into the road.
Some protesters sat on the roadway, while others chanted and yelled at the police from the pedestrian walkway above. Police used orange netting to stop the group on the road from going farther down the bridge, which is under construction.
Some of the protesters said they were lured onto the roadway by police, or they didn't hear the calls from authorities to head to the pedestrian walkway. Police said no one was tricked into being arrested, and those who were in the back of the group were allowed to leave.
"Multiple warnings by police were given to protesters to stay on the pedestrian walkway and that if they took the roadway they would be arrested," said Browne.
Traffic was stopped from 4:30 p.m. until just after 8 p.m. while police removed about 700 protesters. The vast majority, Browne said, were issued a disorderly-conduct summons and released — the equivalent of getting a speeding ticket.
"We may have a few violations for resisting arrest," Browne said.
Browne emphasized that nobody was arrested walking from the park to the bridge or on the walkway.