Tear gas was fired at protesters in Los Angeles on Sunday when some demonstrators moved close to National Guard troops and shouted insults at them, hours after President Donald Trump's extraordinary deployment of the military over the objections of the governor and mayor.

The confrontation broke out as hundreds of people protested in front of the Metropolitan Detention Centre in downtown Los Angeles, where several of the newly-arrived National Guard troops stood shoulder to shoulder behind plastic riot shields.

Video showed uniformed officers shooting off the smoke-filled canisters as they advanced into the street, forcing protesters to retreat. It was not immediately clear what prompted the use of chemical irritants or which law enforcement agency fired them.

Minutes later, loud popping sounds erupted again, as some protesters chanted "go home" and "shame". One person was taken to the ground by uniformed officers. Another appeared to be bleeding from their head.

Around 300 National Guard troops arrived in Los Angeles early on Sunday on orders from Trump, who accused Governor Gavin Newsom and other Democrats of failing to stanch recent protests targeting immigration agents.

The move appeared to be the first time in decades that a state's national guard was activated without a request from its governor, a significant escalation against those who have sought to hinder the administration's mass deportation efforts.

Deployment follows days of protest

The deployment followed two days of protests that began on Friday in downtown Los Angeles before spreading on Saturday to Paramount, a heavily Latino city south of the city, and neighbouring Compton.

As federal agents set up a staging area on Saturday near a Home Depot in Paramount, demonstrators attempted to block Border Patrol vehicles, with some hurling rocks and chunks of cement. In response, agents in riot gear unleashed tear gas, flash-bang explosives and pepper balls.

Tensions were high after a series of sweeps by immigration authorities the previous day, as the week-long tally of immigrant arrests in the city climbed above 100. A prominent union leader was arrested while protesting and accused of impeding law enforcement.

The deployment of the National Guard came over the objections of Governor Gavin Newsom, who accused Trump of a 'complete overreaction" designed to create a spectacle of force.

The recent protests have drawn hundreds of participants but remain far smaller than other mass demonstrations, including the 2020 protests against police violence that spurred Newsom to request assistance from the National Guard.

The last time the National Guard was activated without a governor's permission was in 1965, when President Lyndon B Johnson sent troops to protect a civil rights march in Alabama, according to the Brennan Center for Justice.

Trump says there will be very strong law and order

Trump has framed the move as a necessary response to Newsom's and Los Angeles Mayor Karen Bass's failure to swiftly contain the unrest.

In a directive on Saturday, Trump invoked a legal provision allowing him to deploy federal service members when there is "a rebellion or danger of a rebellion against the authority of the Government of the United States."

He said he had authorised the deployment of 2,000 members of the National Guard.

Trump told reporters as he prepared to board Air Force One in Morristown, New Jersey, Sunday that there were "violent people" in Los Angeles "and they're not gonna get away with it." He said there will be "very strong law and order".

Trump was asked if he plans to send US troops to Los Angeles. He said, "We're gonna have troops everywhere. We're not going to let this happen to our country. We're not going to let our country be torn apart like it was under Biden."

Trump also said that California officials who stand in the way of the deportations could face charges. A Wisconsin judge was arrested last month on accusations that she helped a man evade immigration authorities. "If officials stay in the way of law and order, yeah, they will face charges," Trump said.

Newsom called Trump on Friday night and they spoke for about 40 minutes, according to the governor's office. It was not clear if they spoke Saturday or Sunday.

There was some confusion surrounding the exact timing of the guard's arrival. Shortly before midnight local time, Trump congratulated the National Guard on a "job well done". But less than an hour later, Los Angeles Mayor Karen Bass said troops had yet to arrive in the city.

Defence secretary threatens to deploy active-duty Marines if violence continues

In a statement on 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 (Immigration and Customs Enforcement) officers every single day who wake up and make our communities safer," McLaughlin added.

The troops included members of the California Army National Guard's 79th Infantry Brigade Combat Team, according to a social media post from the Department of Defence.

In a signal of the administration's aggressive approach, Defence Secretary Pete Hegseth also threatened to deploy active-duty Marines "if violence continues" in the region.

Vermont Senator Bernie Sanders said the order by Trump reflected "a president moving this country rapidly into authoritarianism" and "usurping the powers of the United States Congress."

House Speaker Mike Johnson, a staunch Trump ally, endorsed the president's move, doubling down on Republicans' criticisms of California Democrats.

"Gavin Newsom has shown an inability or an unwillingness to do what is necessary, so the president stepped in," Johnson said.