New York riot police launch pre-dawn raid to clear Occupy City Hall protesters

New York police cleared out an "Occupy City Hall" encampment shortly before dawn on Wednesday as mayors across the country moved to prevent federal law enforcement entering their cities.

It comes as violent clashes continued in Portland between protesters and federal officers overnight and the Trump administration threatened to send agents to more major cities.

About 100 uniformed NYPD police carrying riot shields brought the city's month-long Occupy protest to an end about 4 am, with seven people taken into custody.

Commissioner Dermot Shea said six would be released on minor summons and one would be charged with attempted assault for throwing a brick at an officer, who was not injured in the attack.

The decision to dismantle the encampment was made about 10 pm Tuesday by Mr Shea and Mayor Bill De Blasio.

Police issued warnings for 10 minutes before moving into the area with attorneys and cameras. Videos posted online show the officers moving forward in a wall towards tents and tarps as directions were made over a bull horn to the estimated 40-50 protestors that remained at the camp.

By 8 am, city work crews had moved in to begin the clean-up, bringing an end to the occupation that began on 23 June when 100 people set up camp to demand City Council "defund the police".

The area had attracted a growing homeless population, and Mr De Blasio said during a morning press conference that the city does not allow homeless encampments.

"The reality here is that, again, this has become less and less about protest, more and more homeless individuals who were gathered there," Mr De Blasio said.

He said New York was prepared to go to court if the Trump Administration deployed federal law enforcement to the city. "We've seen the situation happening in Portland Oregon, it is deeply troubling," Mr De Blasio said.

New footage from Portland overnight showed the violent clashes at the federal courthouse.

Donald Trump has said federal law enforcement would be sent to more cities with rising crime as local mayors moved to defund police and activists called on supporters to spread the violence seen in Portland to the rest of the country.

Mr De Blasio said on Wednesday he would join 15 Democrat mayors that wrote to acting Homeland Security Secretary Chad Wolf and Attorney General William Barr to pull federal agents out of Portland and not send to more to their cities.

"It is concerning that federal law enforcement is being deployed for political purposes," the letter said.

"The president and his administration continually attack local leadership and amplify false and divisive rhetoric purely for campaign fodder."

Mr Wolf, however, said during a press conference on Tuesday that they would continue to take appropriate action against violent rioters that were not engaged in peaceful protesting.

"We have been forced because of local law enforcement presence to take measures such as arrests to protect our officials," Mr Wolf said.

"They're not military or civilian police officers. These police officers are not stormtroopers. They're not the Gestapo," said Mr Wolf, referring to a remark from House Majority Whip Jim Clyburn.

"As it relates to [Customs and Border Protection] officers deployed in Portland, these highly trained officers are in multi-camouflage wear because they work on the southwest border and they work in an environment that demands that."