The Trump administration has already said it will deploy 2000 US National Guard troops as federal agents in Los Angeles faced off against demonstrators for a second day following immigration raids.
The security agents confronted around 100 protesters in the Paramount area in southeast Los Angeles, on Saturday where some demonstrators displayed Mexican flags and others covered their mouths with respiratory masks.
Hegseth issued the threat to use Marines on X, saying "The @DeptofDefense is mobilizing the National Guard IMMEDIATELY to support federal law enforcement in Los Angeles. And, if violence continues, active duty Marines at Camp Pendleton will also be mobilized — they are on high alert".
Earlier Trump's border czar, Tom Homan, told Fox News that the National Guard would be deployed in Los Angeles on Saturday evening.
California Governor Gavin Newsom called the decision "purposefully inflammatory".
"If Governor Gavin Newscum, of California, and Mayor Karen Bass, of Los Angeles, can't do their jobs, which everyone knows they can't, then the Federal Government will step in and solve the problem, RIOTS & LOOTERS, the way it should be solved!!!" Trump posted on his Truth Social platform.
The protests pit Democratic-run Los Angeles, where a significant portion of the population is Hispanic and foreign-born, against Trump's Republican White House, which has made cracking down on immigration a hallmark of his second term.
In the late afternoon, authorities began detaining some protesters, according to Reuters witnesses. There was no immediate official information of any arrests.
Video footage showed dozens of green-uniformed security personnel with gas masks lined up on a road strewn with overturned shopping carts as small canisters exploded into gas clouds.
"Now they know that they cannot go to anywhere in this country where our people are, and try to kidnap our workers, our people - they cannot do that without an organised and fierce resistance," said protester Ron Gochez, 44.
A first round of protests kicked off on Friday night after Immigration and Customs Enforcement agents conducted enforcement operations in the city and arrested at least 44 people on alleged immigration violations.
Stephen Miller, an immigration hardliner and the White House deputy chief of staff, wrote on X that Friday's demonstrations were "an insurrection against the laws and sovereignty of the United States." On Saturday, he described the day's protests as a "violent insurrection."
The Department of Homeland Security said in a statement about Friday's protests that "1000 rioters surrounded a federal law enforcement building and assaulted ICE law enforcement officers, slashed tires, defaced buildings, and taxpayer funded property".
Reuters could not verify DHS's accounts. Angelica Salas, executive director of immigrants' rights organisation Chirla, said lawyers had not had access to those detained on Friday, which she called "very worrying".
Trump has pledged to deport record numbers of people in the country illegally and lock down the US.-Mexico border, with the White House setting a goal for ICE to arrest at least 3000 migrants per day.
But the sweeping immigration crackdown has also caught up people legally residing in the country, including some with permanent residence, and has led to legal challenges.
Television news footage on Friday showed unmarked vehicles resembling military transport and vans loaded with uniformed federal agents streaming through Los Angeles streets as part of the immigration enforcement operation.
Raids occurred around Home Depot stores, where street vendors and day labourers were picked up, as well as at a garment factory and a warehouse, Salas of Chirla said.
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