Campaigners from Plane Stupid, a group opposed to expansion at Heathrow, parked a van across both lanes of the tunnel and locked themselves to it.
The Met says they and their vehicles have been removed.
Traffic is still affected and passengers are advised to allow extra time to get to Heathrow.
Updates on this story and more on the BBC London live page.
A Heathrow spokeswoman said the earlier demonstration affected traffic around the central terminal area and terminals 2 and 3.
In a statement on its website, Plane Stupid said: "The government needs to choose: build new runways or stop climate chaos. It's that simple."
The airport's spokeswoman said: "Expansion will keep the capital and the whole of the UK at the heart of the global economy, meet environmental targets and balance the needs of local communities."
Five environmental activists have been arrested after blocking a traffic tunnel at Heathrow in protest at plans for a third runway.