Oakland's police chief on Friday defended officers involved in a clash with anti-Wall Street protesters, saying they used what they believed to be the least amount of force possible to protect themselves.

Interim Chief Howard Jordan said he takes full responsibility for the actions of his officers, and promised that allegations of misconduct and excessive use of force would be thoroughly scrutinized.

"I am concerned about the injuries to protesters and officers alike; the decision to use any level of force is never taken lightly, and certainly was not in this situation," Jordan said in a statement.

Officers did not suffer any serious injuries during the clash Tuesday night, just "bangs and bruises from bottles and other objects that were thrown at and on them," said Cynthia Perkins, an assistant to the director at the police department.

Among the protesters injured was Iraq War veteran Scott Olsen, who suffered a fractured skull during the clash. Fellow veterans said Olsen was struck in the head by a projectile fired by police, although the exact object and who might have been responsible for the injury have not been definitively established.

Olsen, whose plight has become a rallying cry at Occupy protests around the world, remained hospitalized in fair condition Friday.

Jordan said he and city staff had reached out to the protesters, as many re-established the camp outside City Hall. Demonstrators at the growing encampment were anticipating an appearance later Friday of left-wing filmmaker Michael Moore.

Meanwhile, in San Diego police arrested 51 people as they cleared out demonstrators who occupied the Civic Center Plaza and Children's Park for three weeks. Dozens of officers and San Diego County sheriff's deputies descended on the encampment around 2:30 a.m., declared an unlawful assembly and removed tents, canopies, tables and other furniture.

Those arrested faced charges of illegal lodging, illegal drug use, unlawful assembly and other charges. Twenty-four people were arrested on charges of blocking officers from performing their duties, police said.

Police said there had been numerous complaints about unsanitary conditions created by human and animal feces, urination, drug use and littering, as well as damage to city property. They said demonstrators may return _ without tents and other belongings _ after the cleanup is finished.

San Diego police Chief William Lansdowne said negotiations with demonstrators had broken down and officers received no cooperation. He said he decided to launch the operation at night because fewer people were there than in the day, when the number of demonstrators has swelled from 75 to about 120.

Demonstrators began arriving at the plaza on Oct. 7 to protest perceived economic injustice and poverty.

San Diego protester Chuck Stemke, a 32-year-old mechanical designer, said he awoke to a loud noise and looked out of his tent to see hundreds of police marching toward him in the darkness. He said he scrambled out and started packing up what he could after demonstrators' lost costly tents a few weeks ago in a similar raid.

"It was very intimidating," he said. "There was a huge show of force."

Similar protests elsewhere have continued in San Francisco, Fresno and Sacramento. The San Francisco camp remained peaceful at Justin Herman Plaza, where 200 to 300 activists have set up a small tent city and only a handful of police officers patrolled the area across from the historic Ferry Building.

More protesters have arrived in the camp since police in Oakland dismantled the encampment across the Bay, said protester Jean Pierre.

"With each march and raid, the camp seems to grow a little more," said Pierre, 26, of Eugene, Ore.

A small group of protesters met a day ago with Mayor Ed Lee, who told them the city officials remain concerned about health and safety at the camp, said protester Phil Oje.

The camp members asked that the police not raid the camp as they did nearly two weeks ago, but Lee made no promises, Oje said.

The protesters planned to march through downtown in costume on Saturday, two days before Halloween.

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Watson reported from San Diego. Associated press reporters Terry Chea and Jason Dearen contributed from San Francisco.