Pockets of protesters clashed with police after thousands of people flooded into central London for a Black Lives Matter demonstration in response to the death of George Floyd.
Activists chanted “black lives matter” and “we will not be silent” in Hyde Park in a peaceful demonstration before tensions escalated later outside Downing Street.
The Metropolitan Police said 13 people were arrested during the protests, which ran into the early hours of Thursday morning.
Star Wars actor John Boyega was among those to speak at the rally before protesters, many wearing masks and holding placards, marched on Westminster.
Mr Floyd died after a white officer held him down by pressing a knee into his neck in Minneapolis on May 25, sparking days of protest in the US.
Prime Minister Boris Johnson said he was “appalled and sickened” to see what happened to Mr Floyd, while chief constables from across the UK issued a joint statement saying they “stand alongside all those across the globe who are appalled and horrified”.
Videos shared on social media showed protesters and police clashing outside Downing Street.
One officer was pushed to the ground in view of the Houses of Parliament, while another clip showed officers being forced down Whitehall by a group advancing towards them.
Footage earlier in the day showed objects including signs and a traffic cone being thrown at police while one protester could be seen being wrestled to the ground and restrained by officers.
Freelance journalist Mattha Busby filmed the moment and said it was “unclear exactly what started things”.
“Police appeared to attempt to take a man from the crowd and pandemonium ensued,” he tweeted.
An online fundraiser for the UK chapter of the Black Lives Matter movement had reached more than £270,000 on Thursday morning, with the funds going to “support black life against institutional racism”.
An online-only rally is due to take place this Sunday, campaign group Stand Up to Racism said, with speakers to discuss “how we turn the new wave of anger over racism and injustice into an effective movement for change”.
Labour MP Barry Gardiner said he broke from social distancing to join the demonstration “and take a knee with thousands of brave young people calling for justice”.
Conservative MP Matt Vickers said while the death of Mr Floyd was “beyond a tragedy”, protest scenes in London were “beyond disgraceful”.
He said: “The police protect us day in and day out and deserve our thanks more than ever, sadly today turned into a shameful opportunity for violence.”
Some officers were seen “taking a knee” earlier in the day, but when protesters knelt at 6pm no police joined them.
At one point, police escorted a man to the side of the road who was bleeding from the head, with blood over his clothes and camera.
A crowd followed a group of officers to outside the Foreign and Commonwealth Office, where protesters shouted “answer for your crimes” and “no justice, no peace”.
Officers formed two lines and rested their batons on their shoulders, shouting “get back” at protesters as they separated the crowds.
A reporter from Nine News Australia was also filmed abandoning his live broadcast to flee as tensions flared up.
Scotland Yard said two men were arrested at Downing Street on suspicion of assaulting an emergency worker and violent disorder.
On Wednesday night, a group of protesters descended on police – who were wearing helmets and holding batons – outside the Cabinet Office, which had been daubed with “BLM” in black paint.
Both plastic and glass bottles were thrown in the direction of officers and protesters pulled down a line of temporary barriers outside Downing Street.
Despite rumours circulating on social media that The Cenotaph had been vandalised during the protests, police said they were “not aware of any damage”.
In the US, protests began in Minneapolis where Mr Floyd died, and quickly spread across the country.
Demonstrations have taken place in areas including New York, Los Angeles, Miami, Columbia, South Carolina and Houston.
Some have included clashes between police and protesters, including the use of tear gas and rubber bullets by officers.
US president Donald Trump has pressed state governors to take a more forceful approach against protesters.