A horse bolted as protesters clashed with police at an anti-racism rally outside Downing Street.
The trouble flared following peaceful Black Lives Matter demonstrations in the capital and in other cities across the country – despite a plea from the Health Secretary for people not to gather during lockdown.
Videos circulating on social media showed missiles being thrown at officers wearing protective gear, with mounted police driving back some of the demonstrators along Whitehall.
One officer appeared to be knocked off their horse, which then bolted – sending crowds of people scattering.
Photographs showed the officer being treated as they lay injured on the pavement, while other images showed bikes being thrown at horses.
The clashes followed a series of marches and demonstrations through London, including outside the US Embassy, in protest against police brutality following the death of George Floyd in Minneapolis.
Elsewhere, boxing heavyweight champion Anthony Joshua used crutches to join people on a peaceful protest in his home town of Watford.
Many people wore face masks and social-distancing measures were encouraged during events in Manchester, Cardiff, Sheffield and Newcastle, among other cities.
At Friday's coronavirus news briefing, Matt Hancock warned people against joining the demonstrations this weekend, pointing out "we're still facing a health crisis and coronavirus remains a real threat".
But people wanted to show solidarity with campaigners in the US and to highlight incidents when black and ethnic minority people in Britain have been victims of racial discrimination and violence at the hands of police and others.
In a speech shared online, 30-year-old Joshua said: "We can no longer sit back and remain silent on this senseless, unlawful killings and sly racism on another human being – based on what? Only their skin colour.
"We need to speak out in peaceful demonstrations – just like today, so well done Watford. We must not use a demonstration for selfish motives and turn it into rioting and looting."
Earlier in London, most demonstrators who gathered in Parliament Square wore masks and face coverings, with some opting for gloves.
Placards carried by demonstrators referenced the coronavirus crisis, with one saying: "There is a virus greater than Covid-19 and it's called racism."
As the rally began, one organiser used a megaphone to tell the crowds: "We are not here for violence. Today is sheer positivity, today is sheer love."
Protester Bobbi, 26, from Chingford, London – who did not give her last name, said: "We're literally living in the history books, we're going to be teaching our future children about this and I want to say I was here to support that."
Thousands of protesters packed central Manchester. They chanted and clapped in unison and held home-made placards bearing the initials BLM.
Several hundred marchers gathered in Newcastle, while thousands more watched an online protest organised in the north-east of England.
Dr Christina Mobley, a lecturer who came to Newcastle University from the University of Virginia in Charlottesville, attended with her five-year-old daughter.
The historian, who is leading the project to decolonise the university curriculum, said: "It is really powerful to see such a young, motivated crowd coming out and organising themselves, handing out masks and working with the police."
She took a photo of one of the police officers who had taken off his helmet during the silence for Mr Floyd.
Meanwhile, an online protest organised by Stand Up To Racism – North East drew an audience of several thousand, who listened to speakers including Janet Alder, whose brother Christopher died in police custody in Hull in 1998.
In Sheffield, hundreds of people gathered on Devonshire Green to protest and hold a minute's silence.
During the gathering, which included speeches, they chanted: "No justice, no peace, no racist police."