Comedian Alistair McGowan is among those protesting against the runway
Plans for a third runway at Heathrow are a "breach of natural justice" because the consultation process was unfair, the High Court has heard.
Local councils, residents and green groups want a judge to quash the expansion plans for the London airport.
Nigel Pleming QC, appearing for the coalition, told the court the government had failed to provide adequate reasons for its decision.
The Department for Transport said it stood by its decision, made last year.
Mr Pleming said: "There was a consultation process here, but the decision made was fundamentally different from the subject matter of the consultation.
"That difference was such as to make it conspicuously unfair."
Village at risk
He asked Lord Justice Carnwath to quash the decision and order a further period of full public consultation.
The QC questioned whether the government should still support its policy when the economic and environmental position had "fundamentally changed" since 2003.
The government approved the third runway and a sixth terminal for the airport in January last year.
The coalition includes six local authorities and campaign groups Greenpeace and the Campaign to Protect Rural England (CPRE).
It will argue over a three-day hearing against the expansion on the grounds of climate change and what it says is a lack of public transport for the development.
Hillingdon Council leader Ray Puddifoot said the government's decision was "fundamentally flawed".
He told BBC Radio 4's Today programme the government had "consulted on one set of proposals" - which the transport secretary then "chose to ignore" - "and decided on another".
He said: "If this airport runs at full capacity... it will produce an extra 23 and a half [million] tonnes a year of CO2 - that's as much as the 54 least polluting countries produce every year, and that's just one airport."
Should the project go ahead, it would mean the disappearance of the village of Sipson, in Hillingdon.
CPRE chief executive Shaun Spiers said: "Proceeding with the third runway would destroy not just a village and a large swathe of green belt, but also tranquillity over a much wider area."
Greenpeace executive director John Sauven said the government had given the go-ahead despite 90% of people who responded to the consultation having been opposed to expansion.
But Lord Soley, campaign director of pro-runway group Future Heathrow, and a former Labour MP, said scrapping the plans would have a "catastrophic" impact on west London.
He said Heathrow provided 72,000 jobs, and most people favoured expansion because their livelihoods depended on it.
He told Today: "So many people's jobs are dependant on that airport remaining a premier hub airport that they do actually support it.
"We have to make it more environmentally friendly because, frankly, people are not going to stop flying."
The Conservatives are against a third runway and shadow transport secretary Theresa Villiers said it had been "only a matter of time" before the "flawed" decision was challenged in the courts.
Liberal Democrat MP Susan Kramer said the "abysmal" expansion plan made no economic sense and would cause "untold" environmental damage.
She said: "It shouldn't take a High Court challenge for the government to see sense and drop its disastrous expansion plan."
A Department for Transport spokesman said: "The department stands fully behind the decisions on Heathrow announced last year and will be defending them robustly in court.
"As matters are currently subject to legal proceedings, it would not be appropriate to comment further."