A French publisher has been arrested in London on suspicion of terror offences over his alleged involvement in anti-pension reform protests in his home country, his employer has said.

Ernest Moret, who works at Editions La Fabrique, was reportedly detained by police after arriving from Paris at St Pancras railway station on Monday evening.

Moret, who had taken the Eurostar and is the publisher’s foreign rights manager, was on his way to the London Book Fair, which takes place in Olympia over three days from April 18-20, when officers tried to “determine whether he was engaged in terrorist acts or in possession of material for use in terrorism”, the firm said.

Officers said he had participated in demonstrations in France over President Emmanuel Macron raising the retirement age from 62 to 64, a joint statement with Verso Books said.

It read: “On arrival at St Pancras Station, Ernest was pulled aside by police officers acting under Schedule 7 of the Terrorism Act 2000 and detained for questioning without a lawyer present, allegedly to determine whether he was engaged in terrorist acts or in possession of material for use in terrorism.

“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 was demanded that he give up his phone and pass codes to the officers, with no justification or explanation offered. This morning, Ernest was formally arrested and transferred to a police station, accused of obstruction because of his refusal to give up his pass codes. He remains in police custody.

“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.

“We consider that this assault on the freedom of expression of a publisher is yet another manifestation of the slide towards repressive and authoritarian measures taken by the current French government in the face of widespread popular discontent and protest.

Youths scuffle with police in Paris during a demonstration in Paris on April 13 (Lewis Joly/AP)

“It is crucial for all defenders of basic democratic values to express in the strongest terms that we find this intolerable and outrageous.”

A protest will take place at the French Institute in south Kensington at 7pm, when people will call for French ambassador Helene Duchene to request Moret’s release.

The National Union of Journalists’ senior books and magazines organiser Pamela Morton urged members to join the protest and said: “It seems extraordinary that the British police have acted this way in using terrorism legislation to arrest the publisher who was on legitimate business here for the London Book Fair.”

The Metropolitan Police and the French embassy in London have been approached for comment.