OAKLAND, California (AP) — Police drove hundreds of anti-Wall Street demonstrators from weeks-old encampments in Portland and arrested more than 50 people, while authorities in Oakland, California, warned Occupy campers that a similar crackdown was coming.

Tension has been building since Sunday night, when Oakland police issued a fourth order telling demonstrators they can’t camp in a downtown plaza and warning of immediate arrest.

Helmet-wearing police from Oakland and several other San Francisco Bay area cities encircled the encampment area at about 4:30 a.m. local time. Some officers held long sticks, while others clutched white zip-ties.

In Portland, police moved in shortly before noon Sunday and forced protesters into the street after dozens remained in the camp in defiance city officials. Mayor Sam Adams had ordered that the camp shut down Saturday at midnight, citing unhealthy conditions and the encampment’s attraction of drug users and thieves.

More than 50 protesters were arrested in the police action, but officers did not use tear gas, rubber bullets or other so-called non-lethal weapons, police said.

After the police raid, the number of demonstrators swelled throughout the afternoon. By early evening, dozens of officers brandishing nightsticks stood shoulder-to-shoulder to hold the protesters back. Authorities retreated and protesters broke the standoff by marching through the streets.

Demonstrators regrouped several blocks away, where they broke into small groups to discuss their future. The Oregonian newspaper reported that numbers began to thin out by mid-evening.

Protesters in Oakland said the main plaza was abuzz with rumors of imminent police action, and campers were discussing what to do and how to safeguard those who decided to stay.

“Oakland is not afraid. We’re not afraid of our tents being taken away, of the movement being stymied,” said Shon Kae, who is part of the group’s media team.

A live video feed posted on the Internet by protesters of the encampment showed many protesters milling as 4 a.m. local time Monday but there were no signs of imminent police action.

The warnings were similar to those issued before officers raided the encampment on Oct. 25 with tear gas and bean bag projectiles. More than 80 people were arrested.

A day later, Mayor Jean Quan allowed protesters to reclaim the disbanded site after facing criticism for her handling of the city’s response, as protesters highlighted that an Iraq War veteran had suffered a serious head injury during the police raid.

On Sunday, friends confirmed that the veteran, Scott Olsen, has been released from the hospital. Olsen, who suffered a skull fracture, became a rallying point for protesters nationwide.

Dottie Guy of Iraq Veterans Against the War said Sunday Olsen was released last week. He can now read and write, but still has trouble talking, she added.

The camp has grown substantially since the Oct. 25 raid, although city officials said on Sunday the number of tents has dropped by about 30 to 150 since Nov. 8.

Officials across the country have been urging an end to similar gatherings in the wake of three deaths in different cities, including two by gunfire.

Demands for Oakland protesters to pack up increased after a man was shot and killed Thursday near the encampment site.

Protesters had said that there was no connection between the shooting and the camp. But police Sunday night identified the slain man as 25-year-old Kayode Ola Foster of Oakland, saying his family confirmed he had been staying at the plaza.

Police officer Johnna Watson said witnesses have told police that one of two suspects in the shooting had also been a frequent resident at the plaza. The suspects are being sought and their names haven’t been released.

Investigators suspect that the shooting resulted from a fight between two groups of men.

In the hours after the midnight Saturday eviction deadline in Portland, the anti-Wall Street protesters and their supporters had flooded the park area. At one point, the crowd swelled to thousands. As dawn arrived, riot police had retreated and most of the crowds had gone home, but protesters who have been at the two parks since Oct. 6 were still there, prompting one organizer to declare the night a victory for the movement.

“We stood up to state power,” Jim Oliver told The Associated Press.

It didn’t last. Police moved in later. An officer on a loudspeaker warned that anyone who resisted risked arrest and “may also be subject to chemical agents and impact weapons.” Demonstrators chanted “we are a peaceful protest.”

City officials erected temporary chain-link fences with barbed wire at the top around three adjacent downtown parks, choking off access for demonstrators as parks officials cleaned up.

Mayor Sam Adams Sunday defended his order to clear the park, saying it is his job to enforce the law and keep the peace. “This is not a game,” Adams said.

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Associated Press writers Terry Collins in Oakland, Josh Loftin in Salt Lake City, Jim Anderson in Denver and Andrew Dalton in Los Angeles contributed to this report.

Copyright 2011 The Associated Press.