The main approach tunnel to Heathrow Airport was blocked by protesters objecting to plans for a third runway.
Three cars were parked across the road, with protesters chaining themselves to one of the vehicles at about 08:30 GMT.
The tunnel, which accesses Terminals 2 and 3, remains closed but the M4 spur road has now reopened after it was shut for about 45 minutes.
Two people have been arrested on suspicion of obstructing a highway, the Metropolitan Police said.
The disruption has caused widespread delays, with traffic being diverted to the outbound tunnel at the airport.
A spokesman for the Rising Up campaign group said it was protesting against the impact a proposed third runway could have on climate change and the local area.
Heathrow Airport said it was working with authorities to resolve the issue.
An airport spokesman 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.”
Campaign group Back Heathrow said the actions of the protesters were “selfish, short-sighted and counter-productive”.