Los Angeles Mayor Karen Bass has issued a curfew for the city's downtown areas "to stop the vandalism, to stop the looting".

She said in a news conference on Tuesday (Wednesday AEST) that she had declared a local emergency and that the curfew will run from 8pm on Tuesday until 6am Wednesday.

The curfew will be in a 2.59 square kilometre section of downtown that includes the area where protests have occurred since last Friday.

"We reached a tipping point" after 23 businesses were looted, Bass said.

The announcement came after US National Guard troops began protecting immigration agents as they made arrests in Los Angeles on Tuesday, an expansion of their duties that had been limited to protecting federal property.

Photos posted by US Immigration and Customs Enforcement (ICE) show National Guard troops standing guard around officers as they made arrests.

ICE said in a statement that the troops were providing security at federal facilities and protecting federal officers"who are out on daily enforcement operations."

The change moves troops closer to engaging in law enforcement actions like deportations as President Donald Trump has promised as part of the administration's immigration crackdown.

Meanwhile California Governor Gavin Newsom has namedDonald Trump a"stone cold liar" as the National Guard continue to support immigration raids throughout Los Angeles.

Speaking to MSNBC, Newsom made the statement in response to Trump's claims he had previously said he would send the National Guard in during a late-night Friday phone call with Newsom.

Newsom said instead Trump had barely wanted to discuss LA, and that it was a"decent conversation".

"He's a stone cold liar," Newsom said.

"I respect the presidency. I've tried to have an open hand with him, tried to work with him. (You) can't work with Donald Trump. You can only work for him.