Public figures in Britain, including the shadow home secretary, Diane Abbott, and the actor Emma Thompson, have signed a letter to the Guardian condemning the conviction of 15 political activists who blocked the takeoff of an immigration removal charter flight.

On Monday, the group, who were members of the End Deportations campaign, were convicted under anti-terror legislation of endangering the safety of Stansted airport during a protest in March 2017.

A jury at Chelmsford crown court heard how they used lock-on devices to secure themselves to a Titan Airways Boeing 767 chartered by the Home Office, as the aircraft waited to remove undocumented immigrants to Nigeria, Ghana and Sierra Leone.

After nearly three days of deliberations, the jury found the defendants guilty of intentional disruption of services at an aerodrome. They were found guilty under the 1990 Aviation and Maritime Security Act, a law passed in response to the 1988 Lockerbie bombing.

In an open letter, more than 300 signatories call for the protesters – known as the “Stansted 15” – to be spared prison, and on the UK government to stop its “inhumane hostile environment policies and to end its barely legal and shameful practice of deportation charter flights”.

Among those who have signed the letter are Abbott and Thompson, the shadow attorney general, Shami Chakrabarti, the author Philip Pullman, the hip-hop artist and writer Akala, the writer Alice Walker, the American political activist Angela Davis, the musicians Brian Eno and Peter Gabriel, and the TSSA general secretary, Manuel Cortes.

“[The Stansted 15] did not aim to inconvenience people, and this arduous trial has had such a profound impact on the defendants’ lives that they are unlikely to take such action again,” the letter says. “The Stansted 15 are a credit to human rights. They and their intentions should be supported, not punished.”

The letter claims that as a result of the group’s 10-hour blockade, 11 people – including victims of trafficking recognised under the Modern Slavery Act – were still in the UK with their loved ones. “This group of ordinary people took extraordinary and peaceful action in the knowledge that they were saving lives,” it says. “But now they face prison sentences.”

It adds: “The UK is the only country in Europe to lock up people with uncertain immigration status indefinitely. 3,000 people are incarcerated in immigration prisons in the UK at any one time.”

Emma Thompson said: “I am inspired and grateful to the Stansted 15 for trying to save lives in this incredibly brave way. They deserve not punishment but our support and our gratitude for fighting for human rights in a country that seems determined to crush them.”

Emma Thompson:’They deserve not punishment but our support and our gratitude.’ Photograph: Getty Images, Rex

On Facebook, more than 1,000 people have said they are attending a demonstration against the convictions outside the Home Office from 5.30pm on Tuesday night.

At the end of the nine-week trial, the judge Christopher Morgan told the jury to disregard all evidence put forward by the defendants to support the defence that they acted to stop human rights abuses. He instructed the jurors to consider only whether there was a “real and material” risk to the airport.

The activists are: Alistair Tamlit, Benjamin Smoke, Helen Brewer, Lyndsay Burtonshaw, Nathan Clack, Laura Clayson, Mel Evans, Joseph McGahan, Jyotsna Ram, Nicholas Sigsworth, Edward Thacker, Emma Hughes, May McKeith, Ruth Potts and Melanie Strickland. They are aged 27 to 44 and all pleaded not guilty.

They will be sentenced at a later date.