A public inquiry has begun into airport company BAA's plans to expand the capacity of Stansted Airport in Essex.
Protesters on the opening day of the public inquiry
BAA wants to increase passenger numbers from 25 million to 35 million each year.
Plans were rejected by Uttlesford District Council so a final decision will now be made at the inquiry.
The Stop Stansted Expansion campaign group, which opposes the plans, staged a protest opposite Endeavour House where the inquiry is taking place.
BAA also wants permission to increase the number of air traffic movements permitted from the airport from 241,000 to 264,000 a year.
BAA said: "This is an important day for the future of Stansted Airport, and for the local community too. This independent public inquiry will take its time in the coming weeks to listen carefully to all points of view."
A spokesman for the Stop Stansted Expansion campaign told BBC News: "If Stansted were permitted to expand to maximum use of the existing runway, the local environment would suffer, the national economy would suffer and we would have taken a giant step backwards in the battle to combat climate change."
Carol Barbone, from the campaign group, said the expansion would not help the economy because it was encouraging people to go on holiday abroad.
The group said BAA's proposal of an extra 10 million passengers annually would create the same amount of additional carbon dioxide as would be saved if everybody in the UK switched from conventional to low energy light bulbs.
BAA said: "The fact remains that more people want to fly to more destinations, more often than ever before, and we are very confident of the case we will be presenting that says we can achieve growth in a sustainable way.
"There are huge social and economic benefits to be gained from growth at Stansted and this is an exciting opportunity that should not be lost."
Gary Pearce, from the GMB Union, which represents thousands of Stansted workers, said the union welcomed the inquiry which is due to last until October.
He added: "If Stansted doesn't keep up with recent growth, the large airlines, whose passenger numbers will increase, will go elsewhere, and that will mean job losses and damage to the local economy.
"Expansion means an additional 5,000 jobs at the airport, which will provide work for local families for years to come."
Today's public inquiry relates to BAA's 'Generation 1' expansion plan. BAA is seeking to:
Remove a current 25 million passenger per year limit, as this level is expected to be reached by 2008.
Increase the current annual limit on flights from 241,000 to 264,000.
Allow passenger numbers to rise to around 35 million per year, which BAA forecasts it to reach around 2014