LONDON - Taking a knee, banging drums and ignoring social distancing measures, outraged protesters from Sydney to London on Saturday kicked off a weekend of global rallies against racism and police brutality.

The death at police hands of George Floyd, an unarmed black man in the US state of Minnesota has brought tens of thousands out onto the streets during a pandemic that is ebbing in Asia and Europe but still spreading in other parts of the world.

"It is time to burn down institutional racism," one speaker shouted through a megaphone at a hooting crowd of thousands outside the parliament building in London.

"This is how we take care of each other," she added after urging everyone to pull on a face mask. "This is how we stay alive."

Officials around the world have been trying to balance understanding at people's pent-up anger with warnings about the dangers of a disease that has officially claimed nearly 400,000 lives globally.

Yet tens of thousands of Australians defied Prime Minister Scott Morrison's call to "find a better way", and thousands more in Britain ignored the health minister's warning that the "coronavirus remains a real threat".

"We want justice! We want to breathe!" hundreds chanted in Tunis, as demonstrations convulsing US cities spilled out across the world.

Dressed in black

"Are you sure of your silence," asked a poster of a man laying a pink rose at a memorial set up outside the president's office in Pretoria, South Africa.

In Sydney, aboriginal protesters performed a traditional smoking ceremony at the start of a "Black Lives Matter" protest, which was sanctioned at the last minute after initially being banned on health grounds.

Many held up signs and wore face masks marked up "I can't breathe" -- the words Floyd kept repeating while handcuffed as a policeman knelt on his neck.

One placard simply read "8:46" -- the amount of time the 46-year-old was pinned to the ground by the white officer before his death.

"The fact that they have tried to push us all back and stop the protest, it makes people want to do it even more," said Jumikah Donovan, one of thousands who turned up thinking the Sydney ban was still in place.

Floyd's death came during the spread of a disease that has disproportionately affected black people and ethnic minorities in global centres such as London and New York.

It also came in the throes of a historic economic downturn that has statistically affected the poor and marginalised the most. This confluence, and accompanying outrage at US President Donald Trump's partisan response, has refocused attention on the world's racial divides like few other events since the 1960s.