The defendants, who have become known as the Stansted 15, said they were “guilty of nothing more than intervening to prevent harm”.
But a judge at Chelmsford Crown Court had told the jury that their intentions were not a defence.
Before sending them out to consider their verdicts last week, judge Christopher Morgan said alleged human rights abuses, immigration policy and issues of proportionality did not have “any relevance” to whether an offence had been committed.
All activists, who are members of End Deportations, had pleaded not guilty to intentional disruption of services and the endangerment at an aerodrome under the 1990 Aviation and Maritime Security Act.
In a statement released after the verdict, they said: “We are guilty of nothing more than intervening to prevent harm.
“The real crime is the government’s cowardly, inhumane and barely legal deportation flights and the unprecedented use of terror law to crack down on peaceful protest.
“We must challenge this shocking use of draconian legislation, and continue to demand an immediate end to these secretive deportation charter flights and a full independent public inquiry into the government’s ‘hostile environment’.”
The law used is not part of the Terrorism Acts, but campaigners said it was created in the wake of the 1988 Lockerbie bombing.
A Crown Prosecution Service (CPS) spokesperson said: “The charge used in this case is from the Aviation and Maritime Security Act of 1990 and applies to those who intentionally disrupt service at an aerodrome, regardless of their motivation. It is not a terrorist charge.”
On 28 March 2017, the defendants cut through the perimeter fence of Stansted Airport in Essex and used pipes to lock themselves together surrounding a plane.
The Boeing 767 had been chartered by the Home Office to remove 60 people to Nigeria, Ghana and Sierra Leone, and was stationary on the airport’s apron.
A pilot who saw the activists approaching closed the plane’s doors before calling security, and the ensuing police operation caused flights to be diverted to other airports.
Prosecutors said airport security and police spent hours removing the defendants before they were arrested, and that thousands of air passengers had their journeys delayed.
The nine-week trial heard the defendants believed the deportees were at risk of death, persecution and torture if they were removed from Britain.
Campaigners said the fact that 11 of the people due to be deported on the flight have since been allowed to remain in the UK justified their actions.
One of the men who was on the flight, who has been granted the right to remain in Britain, said he would have missed his own daughter’s birth and been denied the right to appeal the Home Office’s decision.
“There is no doubt in my mind that these 15 brave people are heroes, not criminals,” he added.
“For me a crime is doing something that is evil, shameful or just wrong – and it’s clear that it is the actions of the Home Office tick all of these boxes – and the Stansted 15 were trying to stop the real crime being committed.”
Melanie Strickland, one of the convicted activists, called the verdict “devastating and profoundly disturbing for democracy”.
“It’s the Home Office that should have been in the dock, not us,” she added.
Raj Chada, a partner at Hodge Jones and Allen who represented the defendants, called the use of the law “draconian”.
He said the firm had previously represented 13 people who protested at Heathrow in similar circumstances but were charged under a different law.
“We believe this was an abuse of power by the attorney general and the CPS as they should never have been charged with these offences,” Mr Chada added.
The group has received high-profile support from MPs including Labour’s David Lammy and Caroline Lucas of the Green Party.
An open letter signed by dozens of politicians and academics in September condemned the practice of “secret deportation flights”, which has come into renewed focus following the Windrush scandal.
“For people living in places like Manchester or Birmingham for many years, this is their home, and they know no one in the country they’re being sent to,” the letter said.
“Many will face persecution, harm or death when they arrive, or the widely documented violence and abuse from security contractors on these flights.
“The Stansted action was the first time people protesting against the immigration system grounded a deportation flight in the UK. Several people due to be forced onto the flight were able to stay because of the action, which bought time to hear their applications.”
Judith Reed, deputy chief crown prosecutor for the east of England, said the group caused “significant disruption”.
“Through their actions these defendants intentionally grounded a Boeing 767 and caused significant disruption at Stansted Airport,” she added.
“Fifteen protestors used equipment such as industrial bolt cutters, chains, expanding foam, scaffolding poles and lock box devices to prevent the takeoff of a plane.
“These people placed themselves, the flight crew, airport personnel and police at serious risk of injury or even death due to their actions on the airfield.
“The CPS worked with the police to build a strong case which reflected the criminality of the defendant’s actions, regardless of their motivation.”
The Stansted 15 will be sentenced at a later date.
They are Helen Brewer, 28, of Ferne Park Road, London; Lyndsay Burtonshaw, 28, of Upper Lewes Road, Brighton; Nathan Clack, 30, of Ferne Park Road, London; Laura Clayson, 28, of Brownswood Road, London; Melanie Evans, 35, of Vicarage Road, London; Joseph McGahan, 35, of Path Hill Farm, Reading; Benjamin Smoke, 27, of Rowley Gardens, London; Jyotsna Ram, 33, of Brownswood Road, London; Nicholas Sigsworth, 29, of Ferne Park Road, London; Melanie Strickland, 35, of Borwick Avenue, London; Alistair Tamlit, 30, of Brownswood Road, London; Edward Thacker, 29, of Ferne Park Road, London; Emma Hughes, 38, of Vicarage Road, London; May McKeith, 33, of Vicarage Road, London and Ruth Potts, 44, of Ashton Gate Terrace, Bristol.