Protesters closed the Brooklyn Bridge in one direction, and police arrested hundreds of people Saturday as the Occupy Wall Street protest entered its third week.

The protesters were arrested for standing in the middle of the road and blocking the Brooklyn-bound lanes of the bridge, which more than 130,000 vehicles cross every day.

About 1,500 marchers passed City Hall and approached the bridge just after 4 p.m. They had planned to take the pedestrian walkway, but the mass of protesters created a bottleneck, so some people started to walk into the vehicle lanes. Police officers with bullhorns announced that if the protesters continued to block traffic, they would be arrested.

Protesters who used the Brooklyn Bridge walkway were not arrested, Paul Browne, head spokesman for the New York Police Department, told The New York Times. Those who took over the Brooklyn-bound roadway and impeded vehicle traffic were arrested.

Christopher Dunn, associate legal director of the New York Civil Liberties Union, said he thought protesters in the back of the line might not have been able to hear the warnings, but Browne responded that those protesters had not been arrested.

Witnesses also said police supervisors had walked ahead of the marchers in the vehicle lanes, as though they were actually leading the marchers. But about a third of the way across the bridge, the police took out orange nets and used them to surround and trap protesters, who were then arrested.

It seemed as if they deliberately moved back to allow people onto the roadway, Etan Ben-Ami, a psychotherapist from Brooklyn, told the Daily News.

The marchers had planned to cross the bridge and then hold a rally in Brooklyn Bridge Park.

The protesters have been camped out in Lower Manhattan since Sept. 17. They had initially intended to camp out on Wall Street itself, but when they were blocked by the police, they moved to Zuccotti Park between Trinity Place and Broadway, a few blocks south of the World Trade Center site.

Police Commissioner Raymond Kelly said the NYPD did not plan to forcibly remove the protesters from the park. We see it as our job to make certain that people can demonstrate peacefully, he told Forbes.

But there have been complaints of excessive police force, particularly by officer Anthony Bologna, whom a video captured pepper-spraying protesters this week.