Men in fatigues and carrying semiautomatic rifles were seen around a federal building, where passersby questioned why they were in an area 11 miles (18 kilometers) from the protests against immigration raids.

"Taxpayer dollars could be used for other things," RonNell Weaver told AFP. "Is this really necessary?"

AFP witnessed Marines temporarily detaining one man at the federal building before they handed him over to law enforcement.

The US military would not say why he was detained, despite multiple requests, but the incident appeared to be a minor -- albeit extremely rare -- example of federal troops detaining a US civilian.

Seven hundred Marines -- normally used as crack troops in foreign conflicts -- along with 4,000 National Guard soldiers are tasked with protecting federal buildings, while local police handle protests over Trump's sweeps for undocumented migrants.

An intense legal battle is underway over Trump's authority to deploy troops on US soil as the country braces for widespread protests Saturday, when the Republican will be overseeing a rare large-scale military parade in Washington.

The parade celebrates the 250th anniversary of the US Army but also coincides with Trump's 79th birthday, and will be the first time tanks and other heavy weaponry have rolled through the capital city in three decades.

In response, a "No Kings" movement has sprung up promising to stage protests in more than 2,000 places across the country, including a large demonstration expected in Los Angeles, which organizers say will feature a "20-foot-tall balloon of Trump wearing a diaper."

"Unprecedented" crowds could attend, Los Angeles Police Chief Jim McDonnell told reporters Friday.

Los Angeles County Sheriff Robert Luna, whose deputies are part of a large law enforcement response in the enormous city, urged protesters to behave properly.

"It's a good cause, but we do not want violent agitators out there destroying property or committing acts of violence," he said.

- California v Trump -

In a show of political force, Trump overrode the objections of Democratic Governor Gavin Newsom to deploy California's National Guard.

The president has repeatedly exaggerated the scale of violence, claiming that without troops, Los Angeles would be "burning to the ground right now."

On Thursday, District Judge Charles Breyer ruled Trump's actions were "illegal" and ordered that he return control of the guard to Newsom. Breyer said the LA unrest fell "far short" of the "rebellion" Trump had described.

However, a higher court quickly paused the order pending an appeal hearing with the Trump administration next Tuesday.

The Department of Justice slammed Breyer's ruling as "an extraordinary intrusion on the President's constitutional authority as Commander in Chief."

The dispute mirrors multiple other tussles over Trump's attempts to expand the limits of presidential power -- but is the first to involve troops.

- Hamlet -

Many in Los Angeles are angry about immigration raids carried out as part of Trump's ambition to deport vast numbers of undocumented migrants from the country.

About 100 mostly good-natured protesters gathered Friday evening outside the federal detention center in Los Angeles that has been at the heart of the rallies, ahead of a nightly curfew placed on the downtown area by Mayor Karen Bass.

In a sign of how contained the demonstrations have been, however, those attending a performance of "Hamlet" -- Shakespeare's play about a mad prince -- and other shows at nearby venues were exempt from the curfew.

Outrage at Trump's raids and the use of masked, armed immigration agents backed by uniformed soldiers have also sparked protests in other cities, including San Francisco, New York, Chicago and San Antonio, Texas.

Tensions hiked further Thursday when California Senator Alex Padilla, a Democrat, was handcuffed and forcibly removed from a news conference by Homeland Security Secretary Kristi Noem.

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