US Park Police arrested 31 people in a downtown Washington park, spokesman Sergeant David Schlosser said on Monday, after protesters put up a two-storey wooden structure authorities had declared unsafe.
The arrests on Sunday were the biggest since protesters began to camp in McPherson Square park, not far from the White House, about two months ago. Demonstrators began to assemble the structure for shelter during the winter.
"Our concern yesterday was the structural integrity of the building," Schlosser said.
Fifteen people were charged with crossing a police line and 16 with disobeying a lawful order. There were no injuries or violence.
Portland police arrested 19 people who were trying to occupy a downtown park in the Oregon city. One man was charged with criminal mischief and trespassing for climbing onto the roof of City Hall, police said.
Some 300 people had attempted to gather in Shemanski Park, beginning late on Saturday, and then marched through the city's streets, police said.
Portland Mayor Sam Adams tweeted on Sunday: "This felt like a private dance party in a public space." But he said he would start taking up issues locally that have been defined by the Occupy Wall Street movement nationwide.
Occupy Portland demonstrators said on Monday they would protest bank residential lending practices and rally in support of those arrested.
Protesters, who have been demanding economic justice for average Americans who they say suffer while the government bails out Wall Street firms, have been arrested in recent weeks as officials try to break up their encampments.
In several cities, officials have cited dangerous health and safety conditions in the encampments and the cost of added policing and other security measures in a time of tight budgets.
A vigil is planned on Monday for Darwin Cox, 23, who was found dead over the weekend at the Occupy Denton camp at the University of North Texas campus, according to a statement from the university president.
The cause of death was unclear but there was no evidence of foul play, it said.
Occupy Denton said in a statement that Cox's "lifelong struggle with drug addiction" was suspected of playing a role in his death.
The Occupy movement began in a downtown Manhattan park in New York but protesters were cleared from that site two weeks ago.
More than 300 arrests were made in Los Angeles last week as police cleared an Occupy encampment there.
Oklahoma City protesters got a temporary restraining order from a judge last week to avoid being forcibly ousted.
In Seattle a judge on Friday rejected a bid by activists to block their eviction from a community college, clearing the way for the city to remove them as early as the next few days.
