After a decade of reviews and U-turns, the government is expected to rule in mid-October on whether to build a third runway at Heathrow, Europe's busiest airport, or at its smaller London rival Gatwick.
Expansion plans have been opposed by residents and some lawmakers at both locations.
Protesters lay on the floor of Heathrow's Terminal 2, with many wearing white masks in a so-called die-in to highlight the impact of air travel on climate change and pollution levels.
A woman wearing a mask lay next to a banner which read: "Stay Grounded. No New Runways."
Others took part in a cycle ride nearby, wearing red T-shirts with the message: "No 3rd runway", according to images posted on Twitter.
A Heathrow spokeswoman said the airport continued to operate as normal during the demonstration.
Heathrow has said it will comply with an extended ban on night flights and meet European air quality rules if the project gets the green light.
It has also said it will meet 11 conditions set out by Britain's Airports Commission, including a requirement on air quality which states that new flights will only be permitted if air quality does not breach EU limits.