Environmental campaigners, politicians and residents packed out a rally to say "no, no, no" to the expansion of the UK's biggest airport.
Liberal Democrat leader Nick Clegg was one of the speakers at the event which came as a consultation period into future plans at Heathrow Airport nears an end.
The Government is consulting on plans for a third runway at the west London airport, a possible sixth terminal and greater use of the existing two runways.
Earlier, police arrested a group of Greenpeace activists who climbed on top of an aeroplane after it landed at the airport.
The demonstration saw a banner hung on the tail fin of an Airbus A320 and five protesters were later taken to the Metropolitan Police's Heathrow station. They were later released on police bail until late April pending further inquiries.
Greenpeace claims the Government consultation process which ends on Wednesday was "fixed".
The rally, at Central Hall in London's Westminster, was so popular that organisers said hundreds of people were unable to get in and had to be accommodated in another hall.
Mr Clegg told the audience: "The case for extending Heathrow Airport is so threadbare it's embarrassing."
He said: "Illness, stress, congestion, climate change, how can the Government claim that these add up to a good idea? They add up to a disaster and it's got to be stopped."
The rally was organised by Heathrow's resident's group Hacan and No TRAG (No Third Runway Action Group) and is backed by 14 councils in the Heathrow area.
