The Marines joined National Guard forces already deployed to Los Angeles before nationwide demonstrations expected on Saturday in what might be the biggest backlash to President Donald Trump since he returned to power.
About 200 Marines would protect a federal building in Los Angeles, Army Major General Scott Sherman, who is in command of both the National Guard and Marine forces, said on Friday.
The administration has authorised 700 Marines to be deployed in the city.
It is uncommon for active-duty troops to be used domestically during civil disturbances, the last time being for 1992's Los Angeles riots over the acquittal of police officers who beat Black motorist Rodney King.
The Marines and National Guard deployed in Los Angeles are assigned to protect federal property and personnel and may temporarily detain people, but they are required to turn them over to civilian law enforcement for any formal arrest.
"I would like to emphasise that the soldiers will not participate in law enforcement activities," Sherman said during a briefing.
Reuters witnessed Marines detain one person at the Wilshire Federal Building, where the Marines had been posted.
A Marine suddenly jumped over some benches and across a garden area to chase after a man, grabbed him and held him down until another Marine assisted him, and the man was later handed over to civilians from the Department of Homeland Security.
Asked about the incident, the US military's Northern Command spokesperson said active duty forces "may temporarily detain an individual in specific circumstances".
The detained man, Marcos Leao, 27, an immigrant and a US Army veteran, said he was told to get on the ground after venturing into a restricted area, as he crossed a line of yellow tape to avoid walking around the building.
Leao told reporters he was an army veteran on his way to an office of the Department of Veterans Affairs when he crossed the yellow tape boundary and was asked to stop.
"They treated me very fairly," Leao said, adding that he was told, "Understand, this is a whole stressful situation for everybody, and we all have a job."
Trump's use of the National Guard was allowed to continue, at least for now, after a federal appeal on Thursday paused a lower court ruling to return control of the California National Guard to Governor Gavin Newsom.
The appeals court ruling does not mean it will ultimately side with Trump, as the matter is scheduled for another hearing on Tuesday.
"We saved L.A. Thank you for the Decision!!!" Trump wrote on his Truth Social platform.
The Trump administration "far overreached its authority", the California Attorney-General's office said.
Democrats, including Newsom, have said the use of military force was unnecessary and an example of Trump's authoritarianism.
Cities across the US were bracing for demonstrations on Saturday, when those also opposed to a weekend military parade in Washington are expected to take to the streets.
Organisers of the "No Kings" events, formed as a counter to the military parade, expect about 1800 demonstrations.
"They've defied our courts, deported Americans, disappeared people off the streets, attacked our civil rights, and slashed our services," the group No Kings, which is organising the day of action, wrote on its website.
