Violence broke out as mounted police officers charged at a crowd of protesters at a Black Lives Matter demonstration in central London.
After a day of largely peaceful protests in cities across the country, tensions began to flare as crowds gathered outside Downing Street.
Videos circulating on social media show mounted officers driving back demonstrators along Whitehall, while protesters were filmed throwing plastic and glass bottles towards police.
Officers in helmets and holding shields had formed two lines outside of Downing Street, separating the crowd.
Police then tried to force demonstrators further down Whitehall. Some protesters linked arms and turned their backs to stop themselves from being moved on, urging each other to “stand your ground”.
An officer was knocked from her horse after riding into a traffic light, the Metropolitan Police confirmed. She is currently in hospital receiving treatment for her injuries.
Metropolitan Police Federation chairman Ken Marsh said the officer had “quite nasty” injuries, telling BBC Breakfast on Sunday morning: “She’s stable – she has some quite nasty injuries which she sustained. And the horse is fine.”
Metropolitan Police Commissioner Dame Cressida Dick said on Sunday morning that assaults on police officers were “shocking and completely unacceptable”, adding that 14 police officers had been injured.
Home secretary Priti Patel said violence towards police at protests was “completely unacceptable” and gave officers her “full support in tackling disorderly behaviour”.
Mayor of London Sadiq Khan praised those who protested peacefully on Saturday but said people who became violent “let down the cause”.
He said: “To the thousands of Londoners who protested peacefully today, I stand with you and I share your anger and your pain.
“George Floyd’s brutal killing must be a catalyst for change worldwide.”
He added: “No country, city, police service or institution can absolve itself of the responsibility to do better.
“We must stand together and root out racism wherever it is found. Black Lives Matter.
“To the tiny minority who were violent and threw glass bottles and lit flares – you endangered a safe and peaceful protest and let down this important cause.”
The Metropolitan Police said that, by 5pm, four arrests had been made. All of the suspects are in custody. The arrests were on suspicion of assaulting police, criminal damage, making threats and calling for violence and an incident of dangerous driving near the US Embassy.
The clashes took place after a largely peaceful day of protests that began on Saturday afternoon.
Earlier protesters dropped to one knee and raised their fists outside the US embassy, chanting slogans such as “silence is violence” and “colour is not a crime”.