Up to 1,000 protesters remained near fenced-off parks which the mayor had given demonstrators a deadline of Sunday morning to leave. About 15 demonstrators were reported to have been arrested.
Police moved in shortly before noon and drove 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.
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 chanted “We are a peaceful protest,” “I don’t see no riot here, take off your riot gear” and “The whole world is watching”. The confrontation was generally peaceful, witnesses said.
Police Chief Mike Reese told KGW-TV it had been his plan to take the parks in a peaceful manner. “Our officers have performed exceptionally well,” he said.
The mayor of Portland, Sam Adams, had ordered the camp to be shut down on the grounds that “crime, especially reported assaults, has increased in the area around the camps”.
On Sunday, the mayor defended his order to clear the park, saying it was his job to enforce the law and keep the peace. “This is not a game,” Mr Adams said.
Choya Adkison, 30, said police moved in after giving demonstrators a false sense of calm. They thought they had time to rest, relax and regroup, she said
“Camp was completely vulnerable, completely defenseless” when police moved in, she said. “I’m disappointed that they created a sense of trust by walking away and then completely trampled it.”
On Sunday at an impromptu news conference, the mayor 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.
Officials said that one officer suffered minor injuries when he was hit by some kind of projectile in the leg. Police had prepared for a possible clash, warning that dozens of anarchists may be planning a confrontation with authorities.
Officers seized pieces of cement blocks Friday, saying they were told some demonstrators had plans to use them as weapons against police. They said they believe some demonstrators were building shields and trying to collect gas masks.
Police also seized incendiary devices, gas masks and marijuana on Sunday after stopping three men for speeding on Interstate 5 south of Portland.
The men told police they had left Occupy Portland an hour earlier and were carrying the equipment in anticipation of a confrontation with authorities, the Marion County Sheriff’s Office said.
Police also moved in to evict protesters camping in the cities of Denver, Colorado and Salt Lake City, Utah, on Saturday.
In Denver, authorities forced protesters to leave a downtown encampment and arrested four people for interfering with officers who removed illegally pitched tents, said police spokesman Sonny Jackson.
In Salt Lake City, police arrested 19 people Saturday when protesters refused to leave a park a day after a man as found dead inside his tent at the encampment. The arrests came after police moved into the park early in the evening where protesters had been ordered to leave by the end of the day. About 150 people had been living in the camp there for weeks.
On Friday, a 42-year-old man was found dead inside his tent at an Occupy Salt Lake City protest in Utah. Officials believe he died from a combination of drugs and carbon monoxide poisoning.