Men in fatigues and carrying semi-automatic rifles were seen around a federal building, where passers-by questioned why they were in an area 11 miles (18 kilometers) from the epicenter of protests against immigration raids.
The men -- normally used as crack troops in foreign conflicts -- are part of a group of 700 Marines and 4,000 National Guard soldiers tasked with protecting federal buildings, while local police handle protests over Trump's sweeps for undocumented migrants.
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.
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.
In a show of political force, Trump overrode the objections of Democratic Governor Gavin Newsom to deploy California's National Guard.
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.
The Department of Justice slammed Breyer's ruling as "an extraordinary intrusion on the President's constitutional authority as Commander in Chief."
'Voices for the people'
Outrage at Trump's raids and the use of masked, armed immigration agents backed by uniformed soldiers has already 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.
The White House hit back, claiming -- despite video evidence to the contrary -- that Padilla "lunged toward Secretary Noem."
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