"Massive" arrests after curfew announced in Los Angeles, (Texas deploys National Guard)

The Los Angeles Police Department announced "massive arrests" in the city center, where people gathered despite a nighttime curfew following days of protests that US President Donald Trump described as an invasion by a "foreign enemy."

Looting and vandalism erupted in the second-largest US city after violence erupted overnight in what had initially been a peaceful demonstration against the detention of immigrants.

"Several groups continue to gather on First Street between Spring and Alameda. These groups are being dealt with and extensive arrests are being made. A curfew is in place," the Los Angeles Police Department said on Twitter.

This came hours after Mayor Karen Bass announced, in statements to reporters, "a local state of emergency and a curfew in downtown Los Angeles to stop vandalism and looting."

A 2.5-square-kilometer area of ​​the city's more than 1,200-square-kilometer area will remain closed until 6:00 a.m. (1:00 p.m. GMT) to everyone except residents, journalists, and emergency personnel, according to the mayor.

A protester told AFP that the unrest stemmed from the arrests of immigrants in a city with a large foreign-born and Latino population.

"I think they're doing it for safety," she said of the curfew. "But I don't think the peaceful protests are part of the problem. It's what's happening on the other side that's inciting the violence."

Small, largely peaceful demonstrations that have turned increasingly violent have erupted in Los Angeles on Friday amid anger over increased immigration arrests.

Thousands of people took to the streets in a number of demonstrations, but smaller groups set fires overnight and smashed windows.

On Monday night, 23 businesses were looted, according to police, who added that more than 500 people have been arrested in recent days.

Protests also took place in cities across the country, including New York, Atlanta, Chicago, and San Francisco.

Trump ordered the deployment of 4,000 National Guard troops to Los Angeles, along with 700 Marines, as part of what he called a necessary escalation to control the situation in the city, despite local law enforcement insisting they were capable of controlling the situation.

A military spokeswoman said the troops would be deployed to the streets later Tuesday or Wednesday.

Their mission will be to protect federal facilities and accompany "federal officers in immigration enforcement operations to provide them with protection."

Protesters told AFP that the troops should be "respected" because they had not chosen to come to Los Angeles, but Lisa Orman denounced what she called a "ridiculous" move.

"The idea of ​​the Marines coming here is just a show. The president wants a big show," she said.

The Pentagon announced that the deployment will cost taxpayers $134 million.

Photos released by the Marine Corps showed uniformed men using riot shields to practice crowd control at Naval Weapons Station Seal Beach.

Los Angeles went about its business as usual, with Hollywood Boulevard packed with tourists, tens of thousands of children heading to school, and traffic jams throughout the streets.

But at a military base in North Carolina, Trump painted a grim picture of the situation.

"What you are witnessing in California is an all-out assault on peace, public safety, and national sovereignty," he told soldiers at Fort Bragg.

He added, "This chaos will not continue. We will not allow a foreign enemy to invade and occupy an American city."

California Governor Gavin Newsom, a Democrat who has previously clashed with Trump, responded by calling Trump's militarization of the city the behavior of "a tyrant, not a president."

He explained that "sending war-trained fighters into the streets is unprecedented and threatens the very essence of our democracy."

In a livestreamed address, Newsom described Trump as "a president who wants no restraint from any law or Constitution, carrying out a unified assault on American traditions."

He continued, "California may be the first, but it's clear that this isn't the end of it."

Newsom filed a petition with the Northern District of California seeking an injunction prohibiting the use of military forces for policing.

Rachel Vanlandingham, a professor at Southwestern Law School in Los Angeles and a former lieutenant colonel, told AFP that Trump's use of the military is "extraordinarily rare" for an American president.

US law largely prohibits the use of the military as a police force, except in the case of a declaration of rebellion, which Trump raised on Tuesday.

University of Missouri professor Frank Bowman said Trump "is trying to use emergency declarations to justify deploying the National Guard first and then mobilizing the Marines."