Oakland, Calif. ? Police were in the process of arresting about 100 Occupy protesters for failing to disperse Saturday night, hours after officers used tear gas on a rowdy group of demonstrators who threw rocks and flares at them and tore down fences.

Police Sgt. Christopher Bolton said the arrests came after protesters marched through downtown Oakland a little before 8 p.m. Saturday, with some of them entering a YMCA building.

Meanwhile, about 100 police officers surrounded City Hall while others were swept the inside of the building to see if any protesters broke in.

More help from other police agencies was also on the way, with busloads of Alameda County sheriff’s deputies arriving in the downtown area late Saturday.

The nighttime arrests came after 19 people were taken into custody in Occupy Oakland protests hours earlier.

Police used tear gas and “flash” grenades on the group Saturday afternoon after some demonstrators threw rocks and other objects at them. Police said three officers were hurt, but they released no details.

Police said the group assembled at a downtown plaza Saturday morning, with demonstrators threatening to take over the vacant Henry Kaiser Convention Center. The group then marched through the streets, disrupting traffic.

The crowd grew as the day wore on, with afternoon estimates ranging from about 1,000 to 2,000 people.

The protesters walked to the vacant convention center, where some started tearing down perimeter fencing and “destroying construction equipment” shortly before 3 p.m., police said.

Police said they issued a dispersal order and used smoke and tear gas after some protesters pelted them with bottles, rocks, burning flares and other objects.

Most of the arrests were made when protesters ignored orders to leave and assaulted officers, police said. By 4 p.m., the bulk of the crowd had left the convention center and headed back downtown.

The demonstration comes after Occupy protesters said earlier this week that they planned to move into a vacant building and turn it into a social center and political hub. They also threatened to try to shut down the port, occupy the airport and take over City Hall.

In a statement Friday, Oakland City Administrator Deanna Santana said the city would not be “bullied by threats of violence or illegal activity.”