Oakland Police on Monday morning moved into an anti-Wall Street protester encampment in the city, clearing out occupants and taking down tents, witnesses said.
The moves were part of an expected clearing operation.
Before the camp was completely cleared of protesters, police took away at least 12 people, as a crowd of protest supporters outside shouted "Shame on you!"
There were further arrests later.
In the action, dozens of police in riot gear took down more than 100 tents, lit by a searchlight from a helicopter overhead, as a separate line of officers kept people from entering the camp.
When the operation was finished, collapsed tents and debris lay scattered throughout the camp.
Police spokeswoman Johnna Watson told reporters that the tent area was now a crime scene and to keep off the campsite.
"We don't want anyone to go through where we have to document property," she said.
The police had also set up makeshift fencing around a plaza facing protesters, some 200 of whom were in the street and chanting at an intersection in the early morning hours.
Oakland has seen several clashes between police and protesters in recent weeks, with numerous arrests and some serious injuries. One major protest temporarily shut down the city's port.
The Oakland group has been among the most visible and active in the nationwide Occupy movement, which started in New York in September and is opposed to what the demonstrators see as an unfair concentration of wealth in the United States.
Among other issues, they object to corporate excesses and bailouts of major banks.
The city of Oakland put out a notice addressed "Dear Business Leader" on Monday morning saying the police were enforcing an order issued on Friday.
It said "the City could not assure adequate public health and safety in the plaza" the protesters were occupying.
The statement also said: "We have instructed City staff working around Frank Ogawa Plaza this morning to delay their arrival ... to 10 am. We are sharing this information with Downtown Businesses so that you can use discretion on whether to delay your work day based on your assessment of the situation."
A rapid transit station near the plaza was reportedly closed.
The Oakland action is one of several in recent days aimed at clearing protesters out of encampments authorities say have become dangers to public health or sources of rising crime.
The weekend saw police clearing operations in Portland, Oregon; Salt Lake City, Utah and Denver as well as threats of action in other cities if protesters did not clear out on their own.