Members of the National Guard faced off with protesters at Los Angeles on Sunday, firing tear gas at a growing crowd that gathered outside a federal complex hours after the federal troops arrived in the city on President Trump’s orders.

The confrontation broke out in front of the Metropolitan Detention Center in downtown Los Angeles, as a group of demonstrators shouted insults at members of the guard lined shoulder to shoulder behind plastic riot shields. There did not appear to be any arrests.

Around members 300 National Guard troops arrived in Los Angeles early Sunday following clashes in recent days between protesters and federal immigration agents. President Trump deployed some 2,000 California National Guard troops over the objections of Governor Newsom Saturday after a second day of clashes between protesters and federal immigration authorities in riot gear.

In a social media post late Sunday Mr. Trump said his administration is acting to “liberate Los Angeles from the migrant invasion.”

Confrontations broke out on Saturday near a Home Depot in the heavily Latino city of Paramount, south of Los Angeles, where federal agents were staging at a Department of Homeland Security office nearby. Agents unleashed tear gas, flash-bang explosives and pepper balls, and protesters hurled rocks and cement at Border Patrol vehicles. Smoke wafted from small piles of burning refuse in the streets.

Tensions were high after a series of sweeps by immigration authorities the previous day, including in LA’s fashion district and at a Home Depot, as the weeklong tally of immigrant arrests in the city climbed past 100. A prominent union leader was arrested while protesting and accused of impeding law enforcement.

Mr. Newsom, a Democrat, said in a post on the social platform X that it was “purposefully inflammatory and will only escalate tensions.” He later said the federal government wants a spectacle and urged people not to give them one by becoming violent.

In a signal of the administration’s aggressive approach, Defense Secretary Hegseth threatened to deploy the U.S. military. “If violence continues, active-duty Marines at Camp Pendleton will also be mobilized — they are on high alert,” Mr. Hegseth said on X.

The director of the FBI, Kash Patel, one of the administration’s most vocal supporters of pardoning the January 6 rioters who assaulted police officers at the U.S. Capitol, promised the Los Angeles miscreants that if they “hit a cop” they are “going to jail.”

“Doesn’t matter where you came from, how you got here, or what movement speaks to you,” he said. “If the local police force won’t back our men and women on the thin blue line, we at the FBI will.”

In a statement Sunday, Assistant Homeland Security Secretary Tricia McLaughlin accused California’s politicians and protesters of “defending heinous illegal alien criminals at the expense of Americans’ safety.”

“Instead of rioting, they should be thanking ICE officers every single day who wake up and make our communities safer,” McLaughlin added.

Mr. Trump’s order came after clashes in Paramount and neighboring Compton, where a car was set on fire. Protests continued into the evening in Paramount, with several hundred demonstrators gathered near a doughnut shop, and authorities holding up barbed wire to keep the crowd back.

Mr. Trump federalized part of California’s National Guard under what is known as Title 10 authority, which places him, not the governor, atop the chain of command, according to Mr. Newsom’s office.

The president’s move came shortly after he issued a threat on his social media network saying that if Mr. Newsom and Los Angeles Mayor Karen Bass did not “do their jobs,” then “the Federal Government will step in and solve the problem, RIOTS & LOOTERS, the way it should be solved!!!”

In 2020, Mr. Trump asked governors of several states to deploy their National Guard troops to Washington, D.C., to quell protests after George Floyd was killed by Minneapolis police. Many agreed and sent troops.

Mr. Trump also threatened at the time to invoke the Insurrection Act for those protests — an intervention rarely seen in modern American history. But then-Defense Secretary Mark Esper pushed back, saying the law should be invoked “only in the most urgent and dire of situations.”

Mr. Trump did not invoke the act during his first term, and he did not do so Saturday, according to Ms. Leavitt and Mr. Newsom.

DHS later said recent ICE operations in Los Angeles resulted in the arrest of 118 immigrants, including five people linked to criminal organizations and people with prior criminal histories.

David Huerta, regional president of the Service Employees International Union, was also arrested Friday while protesting. The Justice Department confirmed that he was being held Saturday at the Metropolitan Detention Center in Los Angeles ahead of a scheduled Monday court appearance.