About 400 people were arrested and three police officers were injured after a weekend protest by members of the Occupy movement in Oakland, Calif., turned into a violent confrontation with law enforcement officers that led to an assault on City Hall.

The clashes began about 3 p.m. on Saturday when protesters marched toward the vacant Henry J. Kaiser Convention Center and began to tear down construction barricades, and the violence extended into early Sunday. The Oakland Police Department said in a statement that the crowd was ordered to disperse after protesters “began destroying construction equipment and fencing.”

“Officers were pelted with bottles, metal pipe, rocks, spray cans, improvised explosive devices and burning flares,” the police statement said. Officers responded by firing smoke and tear gas canisters and beanbags, and they initially arrested 20 people.

Several hours later, some protesters broke into City Hall, the police said, although some of the demonstrators said they found the building’s door ajar. Mayor Jean Quan surveyed the damage there on Sunday, and the city administrator, Deanna J. Santana, said at a news conference that the protesters had broken a window and damaged a historic model of the building. Flags were stolen, she said, and one of them was burned in front of City Hall.