New York police clearing Occupy Wall Street

The city mayor's office said on Twitter that the protesters should "temporarily leave and remove tents and tarps" but could return once the park was clear.

Occupy Wall Street was set up in September to protests against economic inequality and had been followed by dozens of protests around the world.

A camp in Oakland, California was cleared overnight on Monday.

The New York Times said that as the operation in Zuccotti Park began at about 01:00 (06:00 GMT), police gave an announcement, saying: "The city has determined that the continued occupation of Zuccotti Park poses an increasing health and fire safety hazard."

Leaflets were also handed out saying protesters would be allowed to return once the clearance had taken place, but not to bring camping equipment.

Occupants were told to "immediately remove all private property" and that they would be arrested if they interfered with the operation, said the notice. Any belongings left behind would be put into storage.

The protesters' live web stream from the park showed angry crowds chanting "all day, all week, Occupy Wall Street" and "the whole world is watching" as police moved into the camp, close to New York's financial district.

"The police are forming a human shield, and are pushing everyone away," protester Rabbi Chaim Gruber told AP.

Police spokesman Paul Browne said most people had begun leaving the park once the order to vacate was given but that a small group of people were refusing the leave.

He said the park was not heavily populated at the time, the Associated Press reports. At least one person was arrested for disorderly conduct.

The city authorities and Mayor Michael Bloomberg have come under pressure from local businesses to shut down the camp, which has numbered about 200 occupants as it nears its two-month anniversary.

The Occupy movement, inspired by the Arab Spring uprisings and economic protest camps in Spain, is calling for a more equal distribution of wealth.

Organisers in the US say most of the country's money is held by the richest 1% of the population and that they represent the other 99%.

They have received widespread support, including from many authority figures, but there have been concerns about safety and hygiene, while critics of the movement say it has failed to suggest a viable alternative economic system.

The New York action comes after police arrested 33 people in Oakland, California as they raided the protest camp in Frank Ogawa Plaza early on Monday morning.

The camp had been marred by recent outbreaks of violence in and around it, including a fatal shooting last week. However, camp residents had said the killing was unconnected to their protest.

Police had declared the plaza a "crime scene" shortly they entered.

Oakland Mayor Jean Quan said she had to act after "repeated violence and this week a murder".

"We had to bring the camp to an end before someone else got hurt."

Oakland police had said they sympathised with the protesters' cause, but urged them to "leave peacefully, with your heads held high, so we can get police officers back to work fighting crime in Oakland neighbourhoods".

A similar raid ended with police in riot gear arresting 50 people in Portland, Oregon on Sunday evening.

Police in a Vermont city have also evicted protesters after a man fatally shot himself last week inside a tent.

A number of other US cities have seen protests camps spring up in the past two months, and the Occupy movement has also spread to Europe, South America and Asia.