The scene at Stansted as police attempt to remove the protesters
Dozens of flights at Stansted airport have been delayed or cancelled after more than 50 protesters occupied a secure area near the runway.
Climate action group Plane Stupid said it started the protest at 0315 GMT by breaching security.
The runway was closed for about two hours, but reopened at 0810 GMT. Delays are continuing and passengers are being advised to contact their airlines.
Airport operator BAA said the protest was "unlawful". Police made 57 arrests.
A spokeswoman for Essex Police said 50 people had been accused of aggravated trespass, three of attempting to gain access to a restricted area, and four of conspiracy to commit a public nuisance.
Ch Supt Ian Gruneberg told BBC News the demonstrators, who were chained together, were "committing a criminal act" and would be dealt with by the courts
'Not a fortress'
A spokesman for Stansted said a total of 56 in and outbound flights had been cancelled, all of them Ryanair.
The airline said the incident was "unacceptable" and demanded an investigation into how it happened.
Stranded Ryanair passengers were advised to go home and re-book, but were warned there was limited availability on all flights for the next three days.
Nick Barton, BAA's commercial director at Stansted, told BBC Radio 4 he hoped the airport would be able to "recover quite well" from the impact of the protest, but delays were likely for the rest of the day.
He defended airport security and said staff had responded quickly.
"[The protesters] were unable to get to the runway, and a second wave of protest was stopped at the fence," he said.
"You should bear in mind, of course, that the runway's about 2,000 acres [8 sq km] in size - it is an enormous area - and we don't intend to run an airport as a fortress."
The BBC's Stephen Chittenden, at Stansted, said protesters had brought concrete blocks and 6ft-high security fencing with them and erected a "stockade" about 50m (164 ft) from the runway. They then chained themselves to the fencing.
One of the activists taking part, Lily Kember, 21, said they had forced their way in using bolt-cutters while the runway was closed for overnight maintenance work.
Ms Kember said she was "incredibly apologetic" that passengers had been affected, but added: "The effects of climate change are going to be monumental."
Plane Stupid said on its website that the action was intended to draw attention to CO2 emissions from the aviation industry.
One protester, whose full name was not given, said: "We're here because our parents' generation has failed us and it's now down to young people to stop climate change by whatever peaceful means we have left.
"We're afraid of what the police might do to us, we're afraid of going to jail but nothing scares us as much as the threat of runaway climate change."
Ryanair told passengers to go home and rebook online
Stuart Meacock, from Peterborough, was due to fly out of Stansted for a trip to celebrate his first wedding anniversary.
He told BBC Radio 5 Live: "It's disgusting. I mean, it's not only breaking all of the safety rules, but it's disrupting everybody's lives.
"I hope they throw the book at them."
Another traveller Jorg Tittel told the BBC: "I am all for fighting for our environment, but these kids have failed already. They are making thousands of travellers angry."
BAA said it would investigate the break-in and where necessary, learn lessons from it.
"We respect people's right to protest within the bounds of the law," it said in a statement. "However any unlawful or irresponsible behaviour aimed at disrupting the smooth operation of the airport is unacceptable."
In October controversial plans for an expansion of Stansted Airport were given the go-ahead by the government.
Airport owner BAA wants to increase passenger numbers from 25 million to 35 million a year and flights leaving the airport from 241,000 to 264,000 a year.
Objectors say an expansion would damage the environment, but some unions said the proposal could bring new jobs.
Matthew Knowles, from the Society of British Aerospace Companies, said the protesters were "ignoring the reality around aviation and climate change".
"Flight numbers have increased at Stansted, but noise nuisance around the airport has actually decreased and aircraft are 70% more fuel efficient than they were 50 years ago," he said.
"The industry has also set itself targets for a further 50% cut in noise and CO2 emissions from 2000 levels in new aircraft by 2050.
Protesters say they broke through the airport perimeter using bolt cutters
"It is time these ill-informed protests stopped."
In Monday's Guardian, Energy Secretary Ed Miliband called for "popular mobilisation" to support politicians trying to pressure world leaders into a deal on climate change amid fears of economic repercussions.
He said there needed to be "a mass movement" along the lines of Make Poverty History.
Jeremy Hinton was due to travel from Stansted to a UN conference on energy and climate change in Poland, but his flight was cancelled.
"It's easy to sympathise with what they're trying to achieve, but [I have] no sympathy with the way they're going about achieving it," he said.
"It can't be done through illegal means and, from a publicity standpoint, I don't think this gives sympathy to the people who have been on the runway this morning."
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