I'm not sure what's going on exactly but here's this:
A picture of the building and a bus on the scene
A leafleat (page 1)
page 2
This seems to be the only livestream on right now
It looks like there's a lot of people on the scene. Inside there seems to be a lot of activity refurbishing the place to establish medical, sleeping, and work spaces. So far the police seem to be few in number and just monitoring.
actually this livestream is up also
Edit:
The building #OccupySF has taken over was a mental health clinic own by the City, empty for 5 years, til today.
from @OperationLeakS, seems to be corroborated.
Edit2: San Francisco Bay Guardian article with some general info
Edit3: Title changed at suggestion of commenter.
Last edit tonight: pictures