Five pro-Palestinian activists are due to appear in court over an attack on an Israeli arms company in Germany, in proceedings their families say could become a “show trial”.

The Berlin-based activists, who are British, Irish, German and Spanish citizens, have been held in pre-trial detention in separate prisons since 8 September. They are alleged to have broken into Elbit Systems, in the city of Ulm in Baden-Württemberg, in the early hours of 8 September, causing hundreds of thousands of euros of damage before calling the police to arrest them.

The Ulm 5 have been charged with trespass, destruction of property and participation in a criminal organisation under section 129 of the German criminal code. The trial is due to start on Monday.

The section 129 charge means authorities consider the accused a threat to society, allowing them to deny bail. Families of the defendants say they have been locked up for up to 23 hours a day and had access to visits, books, phone calls and mail restricted. If found guilty, they face up to five years in prison.

Speaking on behalf of all the defendants, Benjamin Düsberg, a lawyer for Daniel Tatlow-Devally, 32, from Dublin, said he believed the German state was trying to make an example of the five, none of whom has a previous conviction.

The attack on the weapons factory was an action in “defence of others” in trying to obstruct the movement of arms to Israel, he said.

Düsberg, one of eight defence lawyers, said: “We intend to use the proceedings to essentially turn the tables. We want to show that it’s not our clients who should be on the hook, but rather the Elbit bosses, who continued delivering weapons even during the genocide.”

Elbit Systems is the most important land-based weapons supplier to the Israel Defense Forces (IDF). It has been approached for comment on the trial.

Referring to section 32 of the German criminal code, Düsberg said: “Our central argument will be that the actions of our clients there – namely the destruction of laboratory equipment and office equipment – were justified under the grounds of emergency assistance.”

Under this clause, an otherwise unlawful act can be justified if there is no other way to avert imminent harm or attack, he said.

Germany is the second biggest supplier of arms to Israel, after the US. The defence team will argue that as soon as the international court of justice ruled in 2024 that the claim of genocide against Palestinians in Gaza was “plausible”, Berlin should have stopped all deliveries. Israel rejected the ICJ accusation as “outrageous and false”.

According to prosecutors, the group broke in and destroyed office and technical equipment with axes, detonated smoke bombs and defaced with graffiti the exterior of the building.

The five posted a video of their break-in online. They alerted police and waited in the grounds of the weapons factory for the authorities to detain them. The damage allegedly caused was estimated at €200,000 (£173,000) to €1m.

Mimi Tatlow-Golden, the mother of Tatlow-Devally, a philosophy graduate, said she feared the case had a political dimension and the five would “face a show trial” as the German state sought to send a message about the potential penalties of such actions.

She said: “The friends carried out only property damage, at a specific location and with the aim to end a genocide. They did not hide their identities and presented themselves for arrest. They represent no harm to the public. Using section 129 to keep them in detention … before the trial can in my view only be viewed as serving a political purpose.”

Matthias Schuster, another of the defence lawyers, said: “Our clients are not dangerous but [authorities] believe they should be seen as such to justify the strict custody conditions in which they have been held.”

Nicky Robertson, the mother of Zo Hailu, 25, who is being held in a prison in Bühl, Baden-Württemberg, said the “extreme treatment” the group had received felt “like a disproportionate response for property damage”.

Hailu, also a British citizen, was strip-searched on arriving at the prison and forced to wear an adult nappy, Robertson said. “These are people who love the environment and children, who are caring, creative, sporty, decent team players. They’re not a danger to society. Quite the opposite,” she added.

Rosie Tricks, whose 25-year-old sibling, Crow Tricks, another British citizen, is being held at the maximum-security Stuttgart-Stammheim prison, said visits had been restricted to two hours a month. “It’s lovely to see them but knowing Crow as a sociable, bubbly, fun person, the light of our family, it’s really hard to see them in this position,” said Rosie of Crow. “Their health has definitely suffered. They look OK but inside there’s a lot of anxiety and worry.”

The other defendants are Vi Kovarbasic, a 29-year-old German, and Leandra Rollo, a 40-year-old Spanish citizen from Argentina. The five have continued to be denied bail, even after the six-month limit of pre-trial detention passed.

A spokesperson for the Stuttgart-Stammheim court said: “The code of criminal procedure allows, under certain conditions, for the extension of pre-trial detention.”

In a special detention review last month, Stuttgart’s higher regional court “examined these conditions … and ordered the continuation of pre-trial detention for all defendants”, basing its decision “on the existence of a risk of flight, which would not be sufficiently mitigated even by the posting of bail”.

The court spokesperson added: “Due to its size, its state-of-the-art security and media technology, the new courtroom building is particularly well suited to the requirements of state security trials (such as that of the Ulm 5), especially given the anticipated high level of public interest.”

The trial is expected to run until the end of July.