The unprecedented mobilization of Marines in Los Angeles and threats from a Florida law enforcement official to kill protesters took place on the eve of mass demonstrations across the country against the attempt by Donald Trump to establish a military dictatorship in the US.

Marines temporarily detained a civilian protester in Los Angeles as authorities prepare for No Kings nationwide protests against the Trump administration Saturday.

Maj. Gen. Scott Sherman, commander of Task Force 51 who is overseeing the 4,700 troops deployed, said Friday that the Marines have finished training on civil disturbance.

Infantry Marines arrived in LA with minimal training to counter civilians protesting the Trump administration's immigration crackdown.

"I would like to emphasize that the soldiers will not participate in law enforcement activities," the task force leader told reporters.

Marines are joining National Guard troops in Los Angeles on Friday. Marines will guard federal property while the National Guard turns to protecting local law enforcement.

The 700 Marines deployed to Los Angeles by President Donald Trump have not yet hit the streets and are instead on standby and carrying out nonlethal training.