Protesters started marching up the pedestrian walk way over the bridge while others tried to take the traffic lane. For a few minutes officers held the line and then they turned around and led the way up the traffic lane on the Brooklyn Bridge. From what I saw no police told any of the protesters to leave until they created a barricade in front of the march about halfway through the bridge. They then pulled vans and buses up to the back of the group and started arresting everyone.
In total over 700 people were arrested.