"His actions were illegal ... He must therefore return control of the California National Guard to the Governor of the State of California forthwith," US District Judge Charles Breyer wrote of the president in the ruling, seen by AFP.

However, he paused the order until 12 pm local time (1900 GMT) Friday -- and the White House almost immediately launched an appeal that could make it all the way to the Supreme Court.

Trump's decision to federalize the National Guard -- a reserve force -- and deploy them in California over the objection of the state governor was one not taken by a US president since 1965.

The Republican has argued he did so because protests in Los Angeles against immigration raids ordered by his administration had gotten out of control, and Los Angeles was "burning."

But Newsom and local law enforcement have stated repeatedly that there was no need for the deployment, and the protests have been mostly peaceful and contained to a few city blocks -- albeit tarnished by some spectacular violence, including the torching of several cars.

Critics have accused Trump -- who also deployed 700 active Marines to the sprawling city -- of exaggerating the crisis and exceeding his authority to conduct a power grab.

Breyer's 36-page opinion said the violence fell "far short" of the "rebellion" Trump described to justify calling in the guardsmen.