A horse bolted through a crowd of people in central London as protesters clashed with police at an anti-racism rally outside Downing Street.
Four people were arrested in the capital as trouble flared following a peaceful Black Lives Matter demonstration.
It came as similar protests were held across other cities in the UK – despite a plea from the Health Secretary for people not to gather during lockdown.
The London protest was carried out peacefully for much of Saturday afternoon, but disturbances began breaking out at around 6pm.
Scuffles broke out when missiles were thrown at officers wearing protective gear, with mounted police called in to drive some of the demonstrators back along Whitehall.
Video footage appeared to show an officer colliding with a traffic light before their horse ran through a crowd of protesters, sending them scattering.
The Metropolitan Police later confirmed a female officer fell from the horse and had been taken to hospital. Her injuries are not life-threatening.
The Met said the arrests were for assault on a police officer, criminal damage, making threats and encouraging violence – plus an incident of dangerous driving near the US Embassy in London. All four suspects remain in custody.
Officers are also “investigating the full circumstances” of the officer’s fall, but the Met said the horse made its own way back to its stables nearby.
Glass and plastic bottles were thrown towards officers along with flares, while graffiti was daubed on nearby buildings, including the Cabinet Office, and a small “BLM” motif was painted on the Cenotaph.
Writing on Twitter, she said: “Protests must be peaceful and in accordance with social-distancing rules.
“Violence towards a police officer is completely unacceptable at any time.
“The police have our full support in tackling any violence, vandalism or disorderly behaviour. There is no justification for it.”
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.
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.”
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.”
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.
During the gathering, which included speeches, they chanted: “No justice, no peace, no racist police.”