Black Lives Matter protesters lying on the road have brought traffic heading to Heathrow airport to a standstill.
The campaign group called for "nationwide #shutdown" in a Facebook post, with protests also taking place in Nottingham and Birmingham, and later on in Manchester.
The activists have been lying on the ground in a row to block normal traffic from passing, with banners reading: "This is a crisis," and, "Delays inevitable injustice system failure."
Police said a number of people have been arrested at Heathrow and holidaymakers face traffic disruption - but a Heathrow spokesman said the protest does not appear to be causing people to miss flights.
In Birmingham, video footage of the protest near the airport shows police officers pulling demonstrators away from the middle of the road.
In Nottingham, police have been erecting screens at the scene of the protest to prevent distraction for motorists.
The day of action comes just a day after the anniversary of Mark Duggan's death. Duggan, 29, was shot by police attempting to arrest him in Tottenham, north London, in August 2011. The shooting sparked nationwide riots.
An inquest jury decided Duggan was lawfully killed by a police marksman.