The ruling is the latest legal setback to Trump's efforts to militarize Democratic-run areas, which he claims -- contrary to evidence -- are spiraling into lawlessness.

In June, Los Angeles became the first city to have troops on the streets when Trump went over the heads of the Democratic leadership to order 4,000 state National Guard reservists to put down protests over immigration raids.

Local leaders said the relatively minor protests, which affected only a few blocks in America's second largest metropolis, could easily be handled by city, county and state law enforcement, accusing Trump of authoritarian overreach.

While many have now been demobilized, the US military says 100 Guard troops remain deployed "to protect federal property and federal personnel as they enforce federal law."

In his ruling on Wednesday, Senior US District Judge Charles Breyer said control of the federalized National Guard troops must return to California Governor Gavin Newsom.

Newsom, a frequent sparring partner for the president and widely expected to run for the White House in 2028, welcomed the judge's decision.

"Today's ruling is abundantly clear -- the federalization of the National Guard in California is illegal and must end," he said.

"The president deployed these brave men and women against their own communities, removing them from essential public safety operations.

"We look forward to all National Guard service members being returned to state service."

Lawyers for the Department of Justice had argued that troops who were federalized would remain under the president's command as long as he wanted.

Breyer's ruling knocked down that argument.

That "would permit a president to create a perpetual police force comprised of state troops, so long as they were first federalized lawfully," he wrote.

"Defendants' argument for a president to hold unchecked power to control state troops would wholly upend the federalism that is at the heart of our system of government."

The order was stayed until Monday to allow the government the opportunity to appeal.