Ernest Moret, 29, a foreign rights manager for Éditions La Fabrique, was approached by two plainclothes officers at St Pancras station on Monday evening after arriving by train from Paris to attend the London book fair.

He was questioned for six hours and then arrested for refusing to disclose the passcodes to his phone and computer. His treatment was condemned as an attack on the right to demonstrate, amid calls for protests outside the UK embassy in Paris and French Institute in London.

Moret arrived at St Pancras at 7.15pm with his colleague Stella Magliani-Belkacem, the editorial director at the Paris-based publishing house, when he was confronted by the two officers.

French publishers had drafted a joint letter calling for a protest about Moret’s treatment outside the British embassy in France on Tuesday evening.

When the officers began questioning Moret, she called her friend Sebastian Budgen, a senior editor at Verso Books in London, at whose home she and Moret had arranged to stay.

Budgen arranged for a lawyer to visit Moret. The lawyer called Budgen at 1am on Tuesday to confirm Moret had been arrested over his refusal to tell police the passcodes to his confiscated telephone and laptop. He was transferred to a police station in Islington, north London, where he remained in custody on Tuesday.

A joint press release from Verso Books and Éditions La Fabrique condemned Moret’s treatment as “scandalous”.

It said: “The police officers claimed that Ernest had participated in demonstrations in France as a justification for this act – a quite remarkably inappropriate statement for a British police officer to make and which seems to clearly indicate complicity between French and British authorities on this matter.”

It added: “We consider these actions to be outrageous and unjustifiable infringements of basic principles of the freedom of expression and an example of the abuse of anti-terrorism laws.”

The statement said a protest was planned at the French Institute in London and called on France’s ambassador to the UK, Hélène Duchêne, to request Moret’s immediate release.

Budgen said: “It is causing a stink at the London book fair and there’s a big stink in France as well.” He added: “There’s been an increasingly repressive approach by the French government to the demonstrations, both in terms of police violence, but also in terms of a security clampdown.”

Hundreds of thousands of people took to the streets in France last month over Emmanuel Macron’s use of constitutional executive powers to push through an unpopular increase in the pension age. The protests caused King Charles’s planned visit to France, his first overseas tour as monarch, to be postponed.

The Metropolitan police have been approached for comment.