UPDATE: Police have issued another dispersal order, declaring an unlawful assembly anwyhere on Broadway past 14th Street.
UPDATE: I'm getting reports that police have told media they're planning to arrest everyone in the camp.
UPDATE: I just spoke to the Guardian's Yael Chanoff, who was on the scene when police broke out the tear gas. She said it went off at least three or four times, and that a line of riot police was stationed on 16th Street nearby Frank Ogawa Plaza, the site of the encampment which has been renamed Oscar Grant Plaza by Occupy Oakland.
Chanoff said a standoff between police and protesters was continuing near 16th and Broadway, where protesters had taken over an abandoned building (see below for details). They'd erected barricades on either side of the street in an effort to hold onto the building, but soon police arrived in riot gear and blocked off street entrances. A barricade at 15th and Broadway was reportedly set ablaze but the fire was later put out by some protesters using fire extinguishers.
"There's lingering tear gas in the air all throughout the camp," she told me.
There are reports that police have fired rubber bullets at protesters as well. It seems the conflict in the streets is wearing on.
Here's the original report:
Reports from Oakland are that police have moved in with tear gas and flash grenades in the downtown area, following a day of peaceful marches and rallies that drew tens of thousands into the streets and to the Port of Oakand for a General Strike.
Police in riot gear reportedly lined up at 17th and San Pablo and 17th and Broadway a little after midnight and issued a dispersal order to the hundreds of protesters who remained in the streets nearby Frank Ogawa Plaza, declaring it to be an unlawful assembly between the 1500 and 1600 blocks of Telegraph and Broadway. Oakland North on Twitter  is reporting arrests, loud explosions, and a fire in the intersection of 15th and Broadway. Some fear police are trying to retake the Occupy Oakland encampment.
A little over an hour ago, protesters took over an abondoned building at 520 16th Street that previously housed the Travelers' Aid Society and announced they wanted to turn it into a school and a crisis center, named Raheim Brown Community Center and Free School after a police shooting victim.