http://www.eastbayexpress.com/ebx/how-the-black-bloc-occupied-oakland/Content?oid=3036670
Then the group of Black Bloc demonstrators set fires. Their exact motivation for doing so wasn't clear. When Occupy medic Douglas Connor arrived shortly after 1 a.m. and asked what the fire was for, he got a matter-of-fact response from one of the protesters standing guard. "Fires help dissipate tear gas," the protester said. Apparently, they'd all read it somewhere in a protest manual.
By then, the scene had devolved into chaos. Police closed in on the crowd gathered at and around Frank Ogawa Plaza and issued a dispersal order that few people could hear over the clamor. A fire truck arrived to put out the fire. Police fired tear gas and bean bags, hit protesters with batons, and made about one hundred arrests — including Connor, who was standing by in his military fatigues (he'd served two years at Walter Reed and one in Iraq) with red crosses taped on the sleeves, waiting to see if anyone needed medical attention. In fact, several medics were arrested that night, along with members of the National Lawyers Guild, bloggers, people visiting from the Occupy encampments in Sacramento and Los Angeles, students fretting about term papers they had to write the following day, and members of a group who huddled on the ground in front of City Hall, holding up peace signs. Most people were charged with failure to disperse; a few got hit with vandalism charges. It's still unclear whether anyone who was arrested that night was part of the original building takeover.
"I don't think anyone who was wearing all black [got arrested]," Connor said. "The Black Bloc is very wily. They do stuff to instigate, and then when it gets hot, they leave."