Classy group.

(ABC News) — 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.

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