It all started when a group of about 200 marched up from Van Ness and took over the building on Turk Street near Franklin Street. They unfurled banners from the roof and invited others to come inside and join them. They want to make the building an urban commune -- in their words, "to reclaim and liberate it for the people."
"We're offering medical care, we're offering housing to folks, we're going to have food distribution," a spokesperson for the protesters said.
Occupiers began immediately transforming the empty building into a hostel-like environment, designating rooms for use by the public. They quickly tore down the fence that had surrounded the building, which is owned by the Catholic Archdiocese.
Almost all the police left early Tuesday evening and there are only a few officers keeping watch. An unknown number of protesters are inside planning to make the building their new encampment.
Around 5:30 p.m. the corner of Turk and Gough was a dangerous place to be. That's when a protester climbed on the roof and began hurling hunks of metal pipe and bricks over and over again. Several bricks were aimed at the officers on Turk Street, but the only person hurt appeared to be one of the demonstrators who was hit in the face according to police.
The suspect from the roof was taken into custody when he left the building. Police arrested 34-year-old Jesse Nesbit of San Francisco for felony aggravated assault on a police officer. Police won't say if they will let protesters stay in the building.
"Currently we have sufficient staffing to monitor the overall venue tonight. I'm in charge of that and it was my determination that the staffing that I currently have is sufficient to maintain public safety," San Francisco Police Capt. Keith Sanford said.
At 7:30 the dozens of riot police left the area. At least one protester was disappointed that crowds were attacked from the roof.
"I think that it horrible, but that does not represent Occupy," protester Quincey Carr said.
In addition to the felony arrest, another protester was cited for disobeying an officer when he went into the street and tried to pick up one of the bricks that had been thrown. Police are not saying what their next move is going to be.
Other May Day protests in San Francisco
The May Day demonstrations in San Francisco were mostly peaceful. About 200 people marched along Mission Street between the 16th and 24th street BART stations. There was a heavy police presence, as officers lined the route and blocked traffic for the protesters. The marchers chanted in favor of workers' rights and urged the Obama administration to enact immigration reform.
Monday night a group estimated at 50-100 people dressed in black broke away from an Occupy rally in Dolores Park and launched a vandalism spree. They began smashing windows, throwing paint and defacing store fronts and cars between 13th and 16th streets in the Mission District. Tuesday morning, merchants were cleaning up, many shocked and confused.