A 19-year-old man has been shot and killed in Detroit after an unknown gunman opened fire on protesters after days of unrest in the city.

The protester was fatally shot on Friday morning after looting and rioting continued through the night in the city in the state of Michigan, according to a report from Nine reporter Tim Arvier.

Violent protests have now spread to 19 cities in the US, including Minneapolis, Portland, Dallas, Memphis, Las Vegas, Phoenix, Albuquerque, Atlanta, Louisville, Kansas City, Detroit, Columbus, New York, Washington, San Jose, Los Angeles, Boston and Houston.

A garbage bin burns between a police officer and a protester. Photo / AP
A garbage bin burns between a police officer and a protester. Photo / AP

The protests have continued to intensify around the country, after being sparked by the death of George Floyd.

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In Los Angeles, protests have mostly been peaceful but CNN tonight reported looting and violence was taking place in downtown LA. Videos showed protesters running through the streets and fighting with officers as fireworks exploded among the crowds.

Police open fire shooting rubber bullets at demonstrators. Photo / AP
Police open fire shooting rubber bullets at demonstrators. Photo / AP

Violent protests have broken out in Los Angeles in California as protests sparked by the killing of George Floyd intensify across the US.

Reports have emerged of looting of major businesses in the city's shopping precinct with rioters attacking police vehicles, lighting fires and forcing officers to retreat.

Police fired tear gas and rubber bullets at protesters who blocked a major freeway and vandalised police cars. Police have declared the protests an "unlawful assembly" and have urged residents to shelter in their homes.

Demonstrators in Los Angeles. Photo / AP
Demonstrators in Los Angeles. Photo / AP

Demonstrators smashed through the windows of two patrolling California Highway Patrol cars as hundreds of people blocked the Hollywood 101 Freeway earlier on Friday, according to NBC.

Other protesters who'd gathered outside the Los Angeles Police Department surrounded a police squad. The police headquarters was vandalised with anti-police messages.

The LAPD said they were on alert as protesters became increasingly aggressive on Friday. They declared the protests an unlawful assembly, urging locals to stay inside their homes.

A protester screams at police. Photo / AP
A protester screams at police. Photo / AP

"While the vast majority of individuals in Los Angeles have expressed those views in a peaceful manner, we have witnessed an increasing level of violence and property damage committed by small number of detractors," the LAPD said in a statement on Friday.

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"The violence involved dangerous projectiles directed at our people as well as some property damage to businesses in the area.

Fireworks explode in the middle of protesters in downtown Los Angeles. Photo / Twitter
Fireworks explode in the middle of protesters in downtown Los Angeles. Photo / Twitter

"While isolated, if left unchallenged we face the potential of those actions expanding and hurting innocent individuals."

In Atlanta, a disturbing video shared online showed what appeared to be children in military gear armed with guns.

In New York City, protesters have clashed with police for the second night in a row, with officers injured and dozens of people arrested.

Rioters next to a vandalised bus. Photo / AP
Rioters next to a vandalised bus. Photo / AP

Protesters gathered at a rally outside the Barclays Centre in Brooklyn and reportedly hurled bottles, garbage and other items at officers.

An abandoned police van was set alight by the rioters and other police vehicles were also set alight.

Several officers were injured, a police official told the New York Times. An officer was punched in the nose, another lost his teeth and a third was being treated for a dislocated leg. The report said between 50 and 100 protesters had been arrested.

Videos shared online showed officers and protesters in hand-to-hand combat, punching one another on the streets.

As unrest continues to spread, the Pentagon took the rare step of ordering the Army to put several active-duty US military police units on the ready to deploy to Minneapolis, where the police killing of unarmed African-American man George Floyd sparked widespread protests.

Demonstrators shut down the northbound CA-110 freeway on Friday. Photo / AP
Demonstrators shut down the northbound CA-110 freeway on Friday. Photo / AP

Soldiers from Fort Bragg in North Carolina and Fort Drum in New York have been ordered to be ready to deploy within four hours if called, according to three people with direct knowledge of the orders.

Soldiers in Fort Carson, in Colorado, and Fort Riley in Kansas have been told to be ready within 24 hours. The people did not want their names used because they were not authorised to discuss the preparations.