Black Lives Matter protesters brought the main road into Europe's busiest airport to a standstill on Friday as they blocked the traffic entering London Heathrow.
Police said a number of people had been arrested at the west London airport and holidaymakers faced disruption.
Pictures circulating on social media show a small number of protesters blocking the link road from a major motorway to the airport holding a banner reading "This is a crisis".
"Police at Heathrow are dealing with a demonstration," a spokesman for London police said.
"A number of people were reported to be blocking the road leading to the airport. Officers have attended. A number of people have been arrested. One lane has been opened but there is traffic congestion.
"Police continue to deal with the situation at the scene."
A Black Lives Matter protest took place in London last month following the killing of black men by police in the United States, causing hours of traffic disruption in south London.
The organisation said it was calling for a "nationwide shutdown" on Friday in major English cities such as London, Birmingham, Manchester and Nottingham.
Heathrow said security and the smooth running of Britain's main air hub was paramount.
"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," it said.
"We are sorry to those passengers whose journeys are being disrupted and we are working with the authorities to resolve the issue."