Police flooded into the main New York protest square of the anti-Wall Street movement early on Tuesday, ordering demonstrators to leave or face arrest, an AFP correspondent said.
Most of the hundreds of protesters who have been gathered in New York's Zuccotti Park had left by 2am (1800 AEDT) with a small crowd at the centre surrounded by police, the reporter said.
Some of the police, wearing helmets but not full riot gear, shoved groups of protesters into trucks, while others began tearing down several tents and stacking up signs in piles.
"Liberty Square (Zuccotti Park), home of Occupy Wall Street for the past two months and birthplace of the 99% movement that has spread across the country and around the world, is presently being evicted by a large police force," the demonstrators said in a statement.
New York mayor Michael Bloomberg's office urged the protesters to "temporarily" leave the area, saying they could return later.
"Occupants of Zuccotti should temporarily leave and remove tents and tarps. Protesters can return after the park is cleared," the office said on the micro-blogging website Twitter.
The operation came less than a day after riot police dismantled a similar protest camp in California, arresting more than 30 protesters as part of an increasingly tough line taken by authorities against the protests.
The demonstrations against corporate greed and Washington gridlock have seen an eclectic group of mainly young people set up tents in city squares across the country in what some authorities have said is a threat to public safety.
In the second such action in two days on the US West Coast, officers moved in to clear demonstrators camped out in Oakland, across the bay from San Francisco.
Fuelling the sense of turmoil, a top adviser to the mayor of Oakland, where demonstrations have repeatedly descended into clashes in recent weeks, resigned in protest at the clampdown.
Oakland Mayor Jean Quan said the early morning operations were prompted by a recent murder that happened adjacent to the camp.
"The encampment had become a place where we had repeated violence and this week a murder," Quan told reporters.
"We had to bring the camp to an end before someone else got hurt."
But hours later a top legal adviser, civil rights attorney Dan Siegel, announced that he had resigned in protest shortly before the crackdown.
"No longer Mayor Quan's legal adviser. Resigned at 2am. Support Occupy Oakland, not the one per cent and its government facilitators," he wrote on Twitter. His resignation was later confirmed by Quan.
The demonstrations, coming a year ahead of presidential and congressional elections, have brought together a loose coalition of mainly liberal Americans opposed to the "one per cent" of business and political elites.
The demonstrators accuse Washington of enabling the bankers that brought down the American economy in 2008 and have said they are inspired by the Arab Spring revolts that have convulsed the Middle East.