Thousands of activists have defied social-distancing rules to gather in protest at the death in the US of a black man after a white police officer knelt on his neck.
Police watched as huge crowds of people demonstrated outside the US Embassy in Battersea, south London, sitting in the road and blocking it to traffic.
Five people were arrested during the day, three for breaches of Covid-19 social-distancing legislation and two for assault on police, Scotland Yard said. Those arrested, who were between 17 and 25, were taken into custody.
The US has been rocked by nightly protests by tens of thousands of people across many cities after Mr Floyd died in custody in Minneapolis on Monday.
At least 1,600 people have been arrested in 22 cities as demonstrations descended into violence.
In central London, many protesters knelt, and repeated “No justice, no peace” and “Say my name, George Floyd”.
Others held up placards saying “Racism has no place” and “I can’t breathe” – thought to have been among Mr Floyd’s last words to the police officer, who had handcuffed him during his arrest.
It's thought many had travelled to the capital by public transport despite appeals by rail companies for people to stay away, allowing key workers to be socially distanced from others.
It remains illegal to socialise in groups of more than two in public under coronavirus laws, police chiefs had warned on Saturday.
Officers moved the crowds on, and some protesters walked to the gates of Downing Street, where they held up placards calling for justice.
Outside the US embassy, passers-by applauded the demonstrators and drivers sounded their horns in support. Other activists made their way to Grenfell Tower in northwest London.
More protests in the capital are planned for the coming week.
Hundreds of people also joined a march in Manchester to protest at Mr Floyd’s death.
Just before Trafalgar Square filled up, London mayor Sadiq Khan tweeted: “Lockdown has not been lifted. The virus is still out there.” He advised people to stay at home as much as possible, keep a social distance when outside and avoid public transport.
The deputy chief medical officer insisted on Saturday that lockdown rules “apply to all” and that the country was facing a “very dangerous moment” in the coronavirus pandemic.
The UK has one of the highest death rates in the world from Covid-19.
Top scientists advising the government have expressed growing alarm at the lifting of restrictions when 8,000 new cases are confirmed each day.
Professor Robert West said there was a “huge risk” of a spike in coronavirus infection rates when the lockdown was eased, adding: “Because we don’t have track and trace in place, we won’t know whether this easing of the lockdown has caused an increase in infections for some time, by which time it will be well under way, the second peak will be well under way.”
The Rev Sally Hitchiner, associate vicar at St Martin-in-the-Fields church in Trafalgar Square, who saw the protest there, said: “I’m very sympathetic to the issue but also surprised to see the strength of emotion that has gathered people together.
“It’s showing there are people in the UK who care passionately about the situation in the US.
“Clearly they’re not following lockdown and social distancing, but I think there’s a huge amount of passion there, and that’s overriding their concerns.
“It’s an issue that requires passion but at the same time there’s a huge amount of risk in what they’re doing.”
A police officer has been sacked and charged with third-degree murder following Mr Floyd’s death.