Demonstrators blocked traffic outside Heathrow Airport as part of a day of Black Lives Matter protests in the UK on Friday.
Pictures showed protesters lying in the road close to a roundabout on the M4 outside the London airport under a banner bearing the words "This Is a Crisis".
The @ukblm Twitter account said protests had been organised targeting "major transport hubs because the conventional avenues to justice have been shutdown to us".
The Metropolitan police said 10 people had been arrested, including protesters who "locked" themselves together and remain at the scene.
After being partially closed, the road has now been fully opened.
A Heathrow spokesperson said: “Heathrow supports the right to peaceful protest within the law, but the safety and security of our passengers, aircraft and colleagues together with the smooth running of the operation is paramount. We are sorry to those passengers whose journeys are being disrupted and we are working with the authorities to resolve the issue.”
Alexandra Kelbert, one of the organisers of today's demo, told BuzzFeed News four people had been taken into police vans and arrested, but there are still protesters lying down on the road and others linking arms to block the entire road.
"Right now the police are trying to hide the protesters ... They put some screens in front of them to make it more difficult to identify what's going on for passersby,” she said.
Kelbert, who is not at the demonstration but is part of a network of activists who support the Black Lives Matter cause and helped plan today's day of action, said people arrived at Heathrow Airport early this morning, around 8am. “Other cities started earlier," she said. "Birmingham started at 7am, but they got arrested or removed just before 8am."
The protesters have spoken out about the government's treatment of black asylum-seekers, including the detention of women at Yarl's Wood immigration removal centre, where there have been allegations of abuse and rape of detainees.
Kelbert said that was one of the reasons protesters targeted "one of the busiest roads in Europe".
"It's also close to Harmondsworth detention centre and indeed Heathrow Airport, where a lot of people get deported from," she said. "So it's both an economically important junction and politically [and] symbolically significant location."
Demonstrations also took place in Nottingham and Birmingham as part of nationwide #Shutdown protests, five years after the shooting of Mark Duggan, whose death sparked the London riots.
In Birmingham, four women and one man were arrested, although West Midlands police would not confirm they were taking part in a Black Lives Matter protest.
In Nottingham, tram services were disrupted by protesters lying down on tracks in the city centre.
Police said: "Nottinghamshire police is aware of disruption in the city of Nottingham. Officers are currently on scene and are negotiating with a small number of protesters.
"It is believed that the unplanned demonstration is in line with a national #BlackLivesMatter protest. Our priority will remain the safety of everyone involved and to bring the demonstration to a peaceful conclusion."
Rallies are expected to take place in London, Manchester, and Nottingham later this evening.
Kelbert said other cities might also have "more layered actions" throughout the day.
Black Lives Matter UK released a video earlier this week calling the treatment of black people by police and the authorities a "crisis".
The family of Liverpool teenager Mzee Mohammed, one of the people cited in the video, are still seeking answers over his death in police custody.
A series of Black Lives Matter protests have taken place in the UK in recent weeks in solidarity with the US movement after two unarmed black men – Alton Sterling and Philando Castile – were fatally shot by police in separate incidents in the space of two days.
Demonstrators at events in the UK told BuzzFeed News their protests were both to express solidarity with black Americans and to recognise the people of colour who have died in police custody in the UK.