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.