LONDON -- Thousands of people demonstrated in London on Wednesday against police violence and racial injustice following the killing of George Floyd in Minneapolis, which has set off days of unrest in the United States.

In Athens, police fired tear gas to disperse youths who threw firebombs and stones at them outside the U.S. Embassy toward the end of an otherwise peaceful protest by about 4,000 people. No injuries or arrests were reported.

The London demonstration began in Hyde Park, with protesters chanting "Black lives matter," before many of them later marched through the streets, blocking traffic.

Some of them converged on Parliament and the nearby Downing Street office of Prime Minister Boris Johnson. A few scuffles erupted between protesters and police outside the street's heavy metal gates.

Inside, Johnson told a news conference that he was "appalled and sickened" by Floyd's death on May 25 when a white Minneapolis officer, Derek Chauvin, pressed his knee on the handcuffed black man's neck for several minutes.

Earlier, "Star Wars" actor John Boyega, who was born in Britain to Nigerian parents and grew up in south London's Peckham neighbourhood, pleaded tearfully for demonstrators to stay peaceful.

"Because they want us to mess up, they want us to be disorganized, but not today," he said.

Boyega recalled the case of Stephen Lawrence, an 18-year-old black man from southeast London who was stabbed to death in 1993 as he waited for a bus. The case against his attackers collapsed in 1996, and a government report cited institutional racism by the London police force as a key factor in its failure to thoroughly investigate the killing.

"Black lives have always mattered," Boyega said. "We have always been important. We have always meant something. We have always succeeded regardless and now is the time. I ain't waiting."

Police appeared to keep a low profile during the demonstration and the ensuing marches.

Earlier, the U.K.'s most senior police officer said she was "appalled" by Floyd's death and "horrified" by the subsequent violence in U.S. cities.

Metropolitan Police Commissioner Cressida Dick said the London force would "continue with our tradition of policing using minimum force necessary."

While the London protesters expressed solidarity with Americans protesting Floyd's death, many also pointed to issues closer to home. "Racism is a pandemic," said one placard at the London demonstration.

Some of them carried placards saying "Justice for Belly Mujinga," a 47-year-old railway station worker who died of coronavirus in April, weeks after an incident in which she said she was coughed and spat upon by a customer who claimed to be infected.

Her death has come to symbolize the high toll the virus has taken on ethnic minority Britons and front-line workers -- and, for some, social injustice. Police did not bring charges against the man accused of confronting Mujinga, saying an investigation showed he did not infect her and there was no evidence to substantiate a criminal offence.

Johnson, who has sought to cultivate close ties with U.S. President Donald Trump, was asked what he would say to him. He replied: "My message to President Trump, to everybody in the United States from the U.K., is that ... racism, racist violence has no place in our society."

Johnson said people had the right to protest but "I would urge people to protest peacefully, and in accordance with the rules on social distancing."

"Everybody's lives matter, black lives matter, but we must fight this virus, as well," he said.

In other rallies around the world:

  • More than 1,000 people protested in Stockholm despite a ban on gatherings of over 50 people due to the coronavirus, and while they expressed solidarity with U.S. demonstrators, participants were keen to emphasize that racial injustice was a problem in Sweden, too. One sign read "Make racism bad again.. Police said they had to use pepper spray and make one arrest.
  • About 3,000 people rallied in Finland's capital of Helsinki, although they dispersed an hour later when the number of participants exceeded the 500 maximum allowed under virus restrictions.
  • In Cape Town, South Africa, about 20 people gathered at the gates of parliament and held up signs reading "Justice 4 George Floyd and Collins Khosa." Khosa died a month ago after being confronted by soldiers and police in Johannesburg's Alexandra township. Family members say he died after being choked and beaten, but a South African army investigation cleared the soldiers -- a finding the relatives' lawyers say they will challenge.
  • Police cut short a demonstration in the Dutch port of Rotterdam by thousands of protesters when the crowd got too big for coronavirus social distancing measures.

Many of the London demonstrators appeared to ignore social distancing guidelines in the U.K., where people have been told to stay 2 metres (6 feet) apart.

The coronavirus outbreak has exposed divisions and inequalities within the U.K. A government-commissioned report Tuesday confirmed that ethnic minorities in Britain experienced a higher death rate from the coronavirus than whites, but did not provide any recommendations on how to alleviate the risks.

Figures from London's Metropolitan Police also showed that black and ethnic minority Londoners were more likely than their white counterparts to be fined or arrested for breaking lockdown rules barring gatherings or nonessential travel.

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Associated Press writer Gerald Imray in Cape Town, South Africa, and Raf Casert in Brussels contributed.