Marines, in a Rare Move, Briefly Detain Man in Los Angeles

A man running an errand and trying to enter a Veterans Affairs office at a federal building in Los Angeles was briefly detained on Friday by U.S. Marines who have been sent to the city by the Trump administration to quell unrest.

The man was quickly released and the incident appeared to be a minor one. But it was noteworthy in one major way: Federal troops are rarely deployed on American soil and are rarely seen detaining U.S. civilians, even temporarily.

The man, Marcos Leao, 27, was detained by Marines who were protecting the Wilshire Federal Building, about 15 miles west of where the protests have been taking place in downtown Los Angeles. In an interview, he said he was an Army veteran.

Mr. Leao said he tried to duck under yellow caution tape cordoning off a plaza area outside the building. He said he was undisturbed by his brief detention.

“They treated me very fairly,” he said.

Los Angeles has been on edge for a week, with nightly protests downtown in response to the Trump administration’s immigration raids in the region. Other protests have surfaced in surrounding neighborhoods and cities.

The Trump administration’s deployment of Marines, along with National Guard troops, has stoked outrage among protesters and California officials. A federal judge late Thursday temporarily prevented the federal government’s mobilization of the California National Guard. But an appeals court has blocked that ruling for the time being, freeing up National Guard troops to be in the city during a mass demonstration planned for Saturday.

The federal building is the same one where F.B.I. agents forcibly removed Senator Alex Padilla, Democrat of California, from a news conference on Thursday. The 17-story building includes offices for the F.B.I., the Veterans Affairs Department and a federal passport agency.

A 19th-century law, the Posse Comitatus Act, generally makes it illegal to use federal troops for law enforcement on domestic soil.

Defense Department officials have said, however, in explaining their authority to detain people, that soldiers can do so under Title 10 of the United States Code, which lays out the legal basis for the use of U.S. military forces. Officials said that members of the military may take actions to provide for the defense of others and the defense of property, including temporarily detaining an individual. Any person who is temporarily detained, Defense Department officials have said, will be transferred to civilian law enforcement, and federal prosecutors will decide whether to bring any charges.

Chief Jim McDonnell of the Los Angeles Police Department said in a news conference Friday afternoon that he was “unaware” of the detention by the Marines. The chief said that troops had not been involved in any detentions carried out by L.A.P.D. officers.

Chief McDonnell has been critical of the troops’ deployment, saying that their presence created a “significant logistical and operational challenge” for local law enforcement agencies. On Friday, he said that his department had had some contact with military officials. But he also called that contact “minimal.”