KENNEDY SPACE CENTRE, FLORIDA (BLOOMBERG) - President Donald Trump vowed his administration would end what he called "mob violence" in US cities following the death of an unarmed black man at the hands of Minnesota police, blaming leftist groups for clashes with police and property damage around the nation.

"The mobs are devastating the life's work of good people and destroying their dreams," Trump said at Cape Canaveral, Florida, in remarks following the first launch of US astronauts into orbit from US soil since 2011.

"There will be no anarchy," Trump said.

"Civilisation must be cherished, defended and protected. The voices of law-abiding citizens must be heard, and heard very loudly."

Saturday's (May 30) successful rocket launch by Elon Musk's SpaceX, which is carrying two Nasa astronauts to the International Space Station, had served as a chance for Trump to take a victory lap for an electoral promise to reestablish American dominance in space.

It's also a symbolic step; the US is resuming manned spaceflight just as most of the country begins to emerge from lockdowns from the coronavirus pandemic and is in a deep economic downturn.

Trump touted the launch as part of his "America First" agenda.

DEMONSTRATIONS ALL OVER

Instead, the day was overshadowed by demonstrations in Minneapolis, Louisville, Atlanta, Washington, DC and other cities, continuing the political strife and racial division that have accompanied Trump's presidency.

Protesters demanded justice for George Floyd, who died this week after a Minneapolis police officer knelt on his neck for more than eight minutes during an arrest for an alleged counterfeit US$20 bill.

The officer, Derek Chauvin, has been arrested and charged with murder and manslaughter. He is white.

"Radical left criminals, thugs and others, all throughout our country and throughout the world, will not be allowed to set communities ablaze," Trump said. "We won't let it happen."

Trump did not specify how his administration would act against violent protesters.

"The leadership of the National Guard and the Department of Justice are now in close communication with state and city officials in Minnesota," he said, "and we are coordinating our efforts with local law enforcement all across the nation."

FRIDAY'S SKIRMISHES

Even as Trump spoke in Florida, protesters were gathering again outside the White House. Demonstrators in Lafayette Park across from the White House skirmished with the Secret Service on Friday, leading to six arrests and "multiple" injuries among the agency's personnel, it said in a statement.

Earlier on Saturday, the president encouraged his supporters to rally outside his residence as well, inviting a potentially dangerous confrontation.

On his way to Florida, Trump threatened to unleash the "unlimited power" of the US military on demonstrators, ignoring legal barriers to deploying the military within the nation's borders for law-enforcement purposes.

He has also repeatedly needled the Democratic mayor of Minneapolis, Jacob Frey, expressing outrage that protesters in the city were able to enter and burn a police precinct station.

"Those making excuses or justifications for violence are not helping the downtrodden but delivering new anguish and pain," Trump said at Cape Canaveral.

Bad Apples Despite the outpouring of anger from protesters, who argue that Floyd's death was the result of systemic police brutality and racism, Trump defended the "overwhelming majority" of police whom he said are "incredible in every way."

"No one is more upset than fellow law enforcement officers by the small handful who fail to abide by their oath to serve and protect," the president said.

In a series of tweets on Saturday morning, Trump appeared to revel in the potential for violence outside the White House, warning that Friday's protesters would have been met by "vicious dogs" and "most ominous weapons" had they dared to breach the fence around the property.

Washington Mayor Muriel Bowser said on Saturday that the reference to attack dogs was "no subtle reminder to African-Americans of segregationists that let dogs out on women, children and innocent people in the South."

She called the comments "an attack on humanity."

Trump depicted Secret Services agents as eager to battle the demonstrators, and later issued an appeal to his supporters to assemble: "Tonight, I understand, is MAGA NIGHT AT THE WHITE HOUSE???"