Here is a look at some of the legal questions surrounding the move.

- Is it legal? -

Trump relied on a seldom-used law known as Title 10 to send an initial 2,000 National Guard troops to Los Angeles. He has since ordered another 2,000 Guard members and 700 Marines to the Californian city.

National Guard troops are normally mobilized by a state governor and used domestically to respond to natural disasters such as floods or wildfires.

Trump, exceptionally, sent the troops to Los Angeles against the wishes of California's Democratic governor, Gavin Newsom.

The last time a president defied a state governor to deploy the Guard was in 1965, when president Lyndon B. Johnson sent troops to Alabama to protect civil rights protestors.

Title 10 permits National Guard federalization in times of "a rebellion or danger of a rebellion against the authority of the Government of the United States" but does not give the troops the powers to perform domestic law enforcement duties.

The troops deployed to Los Angeles have been used so far only to provide security around federal buildings in the second-largest US city.

Newsom has accused Trump of exceeding his authority by deploying the troops without his green light and has filed suit in federal court seeking to have the deployment declared unlawful.

- Insurrection Act -

Trump would need to invoke the rarely-used Insurrection Act of 1807 to allow troops to expand their current role in Los Angeles, according to legal analysts.

The Insurrection Act gives a president the authority to deploy the military domestically to perform law enforcement duties such as conducting searches and making arrests.

The Insurrection Act was most recently invoked by president George H.W. Bush at the request of the then California governor to help put down riots in Los Angeles in 1992 that followed the acquittal of police officers involved in the beating of a Black motorist, Rodney King.

It was used by president Johnson in 1968 to quell riots that broke out in the nation's capital and other cities following the assassination of civil rights leader Martin Luther King Jr.

- Posse Comitatus -

Using the military domestically to conduct law enforcement activities is normally barred by another law, the 1878 Posse Comitatus Act.

The Insurrection Act lets a president sidestep the Posse Comitatus Act to suppress "armed rebellion" or "domestic violence" and use the armed forces "as he considers necessary" to enforce the law.

William Banks, a professor emeritus of law at Syracuse University, said the Insurrection Act and waiving of Posse Comitatus has been infrequent because of a long US history of "leaving law enforcement to civilians."

"To sum up the conditions where (the Insurrection Act) may be used, it's for when all hell breaks loose," said Banks, co-author of the book "Soldiers on the Home Front: The Domestic Role of the American Military."

"When state and local officials are unable to control civil affairs without federal involvement, the federal government may intervene," he told AFP. "It's normally been requested by the state officials, and the president simply agrees and decides to send a federal force."

Newsom has said repeatedly that there was no need for the deployment of the National Guard and Marines and that the Los Angeles Police Department was fully capable of handling the unrest.