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Up to a thousand protesters were in a standoff with ranks of Portland police officers in riot gear near the fenced-off parks the Occupy movement had been ordered to leave over the weekend.
About 15 demonstrators were arrested, but the confrontation was generally peaceful, observers said.
Face-to-face with the protestors, police repeatedly warned them of arrest or the use of chemical agents and impact weapons if the street were not cleared.
On their side, protesters chanted: "I don't see no riot here, take off your riot gear," "The whole world is watching," and "We are peaceful people."
Officials have also moved over the weekend to dismantle a protest camp in Salt Lake City, Utah, while in Denver, police yesterday removed mattresses, cooking grills and tents illegally placed on a public sidewalk.
Seventeen people were arrested, Denver police said on Sunday.
Encampments sprang up in several cities in recent weeks in solidarity with the Occupy Wall Street movement in New York to protest what demonstrators see as economic inequality and undue influence over US politics by corporate interests.
Citing health and safety issues, some officials have urged demonstrators to take down the camps and others have used such concerns as reasons for police to force the issue.
In Philadelphia, Mayor Michael Nutter today ordered beefed-up police patrols at the city's protest site, saying conditions were "dramatically deteriorating."
"This movement has changed and the people have changed," he said. "We are now at a critical point where we must reevaluate our entire relationship with this very changed group."
He said communication has broken down between officials and protesters, and city concerns over fire hazards, litter and a lack of toilets have gone unaddressed.
Added to that are thefts, assaults and an alleged sexual attack at the camp at Dilworth Plaza, he said in a statement.
The mayor said a $US50 million makeover of Dilworth Plaza is planned and by opting not to move, "Occupy Philly is now purposely standing in the way of a nearly 1000 jobs for Philadelphians at a time of high unemployment."
In Salt Lake City authorities moved to dismantle a camp at a downtown park and arrested 15 people yesterday.
In Oakland, California, the scene of previous clashes between police and demonstrators, city hall issued a third eviction notice today. It warned protesters they faced "immediate arrest" if they continued to camp out in the city's plaza and parks.
The city offered alternative emergency accommodation at two local area homeless shelters, and laid on a shuttle service to one that was not within walking distance of the encampments.
In St. Louis, Mayor Francis Slay has warned protesters they have to decamp but has offered to continue talks to find a permanent place for the protest.
The nationwide protest movement, which started in New York in September, has voiced opposition 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.