US security forces clashed with protesters in Los Angeles on Sunday as National Guard troops deployed by US President Donald Trump fanned out across the city on the third day of unruly protests over federal immigration raids.

Trump, who has made clamping down on illegal migration a key plank of his second term, vowed the troops would ensure "very strong law and order," while appearing to leave the door open to deploying soldiers in other cities.

The US military said 300 Guardsmen from the 79th Infantry Brigade Combat Team had been sent to three separate locations in the greater Los Angeles area, and were "conducting safety and protection of federal property & personnel."

Helmeted troops in camouflage gear and carrying guns were stationed at a federal detention centre in downtown Los Angeles, where they joined Department of Homeland Security forces.

Pepper spray and tear gas were fired into a small crowd - including at journalists - as forces moved protesters back to allow a convoy of vehicles to enter the detention centre.

Police later moved and pushed demonstrators away from the federal lines, using batons, flash-bang grenades and tear gas as they tried to disperse the crowd, some of whom spilled onto a major freeway, stopping traffic.

Trump, asked about the use of troops, hinted at a more widespread deployment in other parts of the country.

"You have violent people, and we are not going to let them get away with it," he told reporters. "I think you're going to see some very strong law and order."

Responding to a question about invoking the Insurrection Act - which would allow the military to be used as a domestic police force - Trump said: "We're looking at troops everywhere. We're not going to let this happen to our country."

The deployment in California - the first over the head of a state governor since the Civil Rights era - was "purposefully inflammatory," Governor Gavin Newsom said.

"We didn't have a problem until Trump got involved. This is a serious breach of state sovereignty - inflaming tensions while pulling resources from where they're actually needed. Rescind the order. Return control to California," he wrote on X.

A joint statement by Democratic governors from across the country in support of Newsom said Trump's use of the California National Guard was "an alarming abuse of power."

The National Guard - a reserve military - is frequently used in natural disasters, and occasionally in instances of civil unrest, but almost always with the consent of local authorities.

Raids by the Immigration and Customs Enforcement (ICE) agency in other US cities have triggered small-scale protests in recent months, but the Los Angeles unrest is the biggest and most sustained against Trump's immigration policies so far. (AFP)