Tens of thousands have taken to the streets of London overnight, rallying for a second day running to condemn police brutality after the killing of George Floyd in Minneapolis, with some wearing face masks bearing the slogan “Racism is a virus”.

On Saturday, thousands gathered in central London in a demonstration that was peaceful but ended with small numbers of people clashing with mounted police near Prime Minister Boris Johnson’s Downing Street residence.

Authorities had urged protesters not to gather in London again on Sunday, warning they risked spreading COVID-19, but demonstrators still packed the road outside the US Embassy on the south bank of the River Thames.

Protesters later marched across the river towards parliament and Downing Street, pausing on the bridge to go down on one knee and chant, “Justice, now!”, while in Parliament Square, many attached their placards to the railings outside parliament.

“Now is the time: we need to do something. We have become so complacent in the UK but the racism that killed George Floyd was born in the UK in terms of colonialism and white supremacy,” said 28-year-old Hermione Lake, who was holding a sign reading: “white silence = violence”.

“We need to completely gut the system … We need massive reform, massive change.”

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London police chief Cressida Dick said 27 officers had been injured in “shocking and completely unacceptable” assaults during anti-racism protests over the past week, including 14 on Saturday.

Two were seriously hurt and an officer who fell from her horse underwent surgery.

The May 25 death of Floyd, an unarmed African American, has sparked demonstrations around the world over police treatment of ethnic minorities. A white police officer detaining him knelt on his neck for nearly nine minutes.

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In Bristol in western England, demonstrators tore down a statue of 17th century slave trader Edward Colston.

Sunday’s London protest was largely peaceful. People waved placards and chanted: “George Floyd!” and “The UK is not innocent!”

As the numbers dwindled, some protesters clashed with police outside the Foreign Office after one man was arrested. They threw bottles and flares and chanted: “Black lives matter!” as they tried to push through a line of riot police.

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Mr Johnson said that while people had the right to protest peacefully, the demonstrations had been “subverted by thuggery”.

“They are a betrayal of the cause they purport to serve. Those responsible will be held to account,” he said on Twitter.

Police said 29 people had been arrested during Saturday’s protest in London for offences including violent disorder.

-AAP