Harry Potter author J.K. Rowling has voiced her support for Star Wars actor John Boyega who made an emotional speech at a Black Lives Matter protest in London.

There is nervousness among the British authorities that the demonstrations in London held in sympathy with the protest movement in America over the death of George Floyd could easily get out of hand.

Thousands turned out in London on Wednesday with clashes with police outside Downing Street — the cabinet office in Whitehall was daubed with “BLM” in black paint — and another demonstration was planned in Birmingham.

In London there have been 13 arrests after the clashes in 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. Other footage showed objects including signs and a traffic cone being thrown at officers, while plastic and glass bottles were also seen being launched in their direction.

There have been a number of black people who have died in police custody over the years, though nowhere near the figure for America.

In 2011, there was widespread rioting and looting in cities across Britain after a 29-year-old black man, Mark Duggan, was shot dead by police in Tottenham, north London. Shops and residential property were set on fire.

This time the violence has so far been contained but the relationship between young blacks and the police, though not as poisonous as it used to be even 20 years, has the potential to get out of hand. An official report by a senior white judge had called the British police “institutionally racist” but there have been any reforms since then.

Boyega, 28, who was born in London to parents of Nigerian descent, observed that he could be punished for speaking out: “I don’t know if I’m going to have a career (in Hollywood) after this.” He addressed a crowd gathered in London’s Hyde Park.

through a megaphone: “I’m speaking to you from my heart.”

“Black lives have always mattered,” said actor best known for playing Finn in the Star Wars sequel trilogy films, The Force Awakens (2015), The Last Jedi (2017), and The Rise of Skywalker (2019).

He added: “We have always been important. We have always succeeded regardless.”

There was a supportive tweet from Rowling: “Happy to work with you any time, John.”

The Prime Minister Boris Johnson, who is known to have liberal leanings, has tried to defuse the tension by stating that he was “appalled and sickened” to see what happened to Floyd.

Though the Duchess of Sussex, the erstwhile Meghan Markle, now lives with her husband, Prince Harry, in Beverly Hills in Los Angeles, she released a video of her visiting her old school.

The mixed race actress spoke of her anguish at witnessing the racial divisions in America and the death of Floyd.

Meghan, now 38, recalled living through the 1992 race riots in LA which were “also triggered by a senseless act of racism”.

“What is happening in our country and in our state and in our home town of LA has been absolutely devastating,” she said.

No one doubts that the grievances in the UK are genuine.

Speaking on BBC Breakfast, poet and musician Benjamin Zephaniah, who is now almost an establishment figure, explained what he felt after Floyd’s death: “I just felt rage. I just wanted to take to the streets.”

Zephaniah, whose cousin Mikey Powell died in police custody in 2003, said: “I have my poetry and my music to express myself - at least I have that.

“But if I was a young man now, I would probably be on the streets too, so I understand how people feel and I tell you whatever country you are in – Australia, Britain, Canada, US – bringing the army on the streets is not going to stop the problem. We want compassion, we want understanding, we want an end to racism.”

Meanwhile, chief constables from across the UK have issued a joint statement saying they “stand alongside all those across the globe who are appalled and horrified”.

Officers were also given permission to bend their knees as demanded by protestors as a sign of respect to Floyd – and some did so.

One feature of the demonstrations in London is blacks have been joined by many whites.

However, the authorities are well aware it would take very little to trigger a breakdown of law and order.