Police have declared all of downtown Los Angeles to be an unlawful assembly area and ordered protesters to go home, after Sunday saw a third day of violence hit demonstrations against President Donald Trump's immigration policy.

National Guard troops arrived in downtown Los Angeles on Sunday after being ordered into the city by President Trump in response to a weekend of violent clashes between law enforcement officers and protesters triggered by immigration enforcement operations in the area on Friday.

Dozens of arrests have been made after three days of protests, with the National Guard now patrolling the city.