Fifteen activists who locked themselves together around an immigration removal charter flight to prevent its departure from Stansted and displayed a banner proclaiming “mass deportations kill” have gone on trial charged with a terrorist offence.

Jurors at Chelmsford crown court heard how the members of the campaign group End Deportations used lock-on devices to secure themselves around the Boeing 767, chartered by the Home Office, as it waited on the tarmac at the Essex airport to remove undocumented migrants to Nigeria, Ghana and Sierra Leone.

The activists have said they acted to prevent human rights abuses from taking place and have received high-profile political backing. However, they are accused of putting the safety of the airport and passengers at risk and causing serious disruption to international air travel. If convicted, they could face potential life imprisonment.

Amnesty International UK is sending representatives to observe the trial owing to concerns that the serious charge has been brought to deter other activists from taking similar non-violent direct action to defend human rights. Kate Allen, its director, likened the use of the law to “using a sledgehammer to crack a nut.”

The protest took place on the night of 28 March 2017The activists cut a hole measuring one square metre in the airport’s perimeter fence, Chelmsford crown court heard. Jurors saw videos from CCTV cameras and a police helicopter that showed how four protesters arranged themselves around the front landing gear of the aircraft and locked their arms together inside double-layered pipes filled with expanding foam.

Further back, a second group of protesters erected a two-metre tripod from scaffolding poles behind the engine on the left wing of an aircraft, on which one perched while others locked themselves to the base to prevent it from being moved, the videos showed. In the moments before police arrived, they were able to display their banners, one of which said: “No one is illegal.”

Tony Badenoch QC, for the prosecution, told the court the protest led to the shutdown of the airport runway for an hour and 20 minutes. Outbound planes were delayed and inbound flights were diverted to other airports across England. It took the removal team until 8am the next morning to free the protesters one by one.

“If the fence is breached, and if suddenly there is access airside in the pitch black to an unknown number of people with an unknown purpose who are not immediately brought under control, the airport cannot operate safely and safety cannot be agreed to runway users,” he said. “It is therefore necessary to close the runway until such time as the situation is clearer, or at the very least contained.”

Badenoch claimed the diversion of armed officers from security duties in the terminal building made the airport more vulnerable at a time when the UK terrorism threat level was classed as severe, meaning an attack was considered highly likely.

“In order to deal with this incursion, a number of armed officers already at Stansted had to down-arm, thus reducing the capacity of police to carry out their duties at the terminal,” he said. “Had another major incident occurred at the terminal at the same time, the police resources able to respond to it would have been reduced.”

Helen Brewer, Lyndsay Burtonshaw, Nathan Clack, Laura Clayson, Mel Evans, Joseph McGahan, Benjamin Smoke, Jyotsna Ram, Nicholas Sigsworth, Alistair Temlit, Edward Thacker, Emma Hughes, May McKeith, Ruth Potts and Melanie Stickland are charged with intentional disruption of services at an aerodrome under the 1990 Aviation and Maritime Security Act, a law passed in response to the 1988 Lockerbie bombing. The defendants, aged 27 to 44, have all pleaded not guilty.

The trial, which is due to last six to eight weeks, continues.