Bar brawls in Spain are typically intemperate moments regretted the next day. But one fight that took place in Navarre, close to the Basque country, is proving different. For eight young people implicated in the punch-up, the hangover has endured for nearly 18 months and their case has become a regional controversy.

A special division of the Spanish high court in Madrid will decide in a trial starting on Monday if their offences deserve to be charged under terrorism laws, which could see them collectively face up to 375 years in prison. The decision to transfer the case to the high court was made by Carmen Lamela – the same judge who jailed Catalonia’s separatist leaders for sedition and rebellion last year. One former supreme court judge, José Antonio Martín Pallín, has called the move “radically absurd”.

The verdict could test the peace that has settled in the Basque country since negotiations between the federal government and separatist movement Eta began nearly 10 years ago. The defendants’ parents admit there was a fight but they question the motives of the prosecutors who have ramped up the charges.

“We’ve seen this before, this injustice and logic of vengeance where the Spanish state is against us,” says Isabel Pozueta, mother of 23-year-old Adur who was involved in the punch-up. “They are using a dynamic of war to make an example of us.” Along with two others, her son has been detained in prison for over a year awaiting trial. He faces up to 50 years in prison.

The fight began around 5am on 15 October 2016 in the tiny town of Alsasua, which is nestled in a mountainous region once fiercely contested by the separatists. Two off-duty civil guards entered a local bar with their girlfriends. They were quickly identified by a group of 20- to 30-year-olds and told they were not welcome.

Soon afterwards, blows were landing. The worst injury was suffered by one of the guards, who fractured an ankle and required surgery. The case hinges on whether this was just a drunken scuffle, or whether, as the prosecution contends, the offences committed justify charges of “terrorism”. The prosecution alleges terrorist intent because some of the accused had been involved with a social movement that advocates the removal of state security forces from Navarre –a position also held by Eta.

But local politicians believe these events are being blown out of proportion for political effect.

“Some political parties need the Eta name to survive as it has served them well in elections in the past. This is irresponsible and a huge step backwards for the peace process,” argues Unai Hualde Iglesias, the deputy regional chief of Navarre.

He blames the prime minister, Mariano Rajoy, in particular for “aggravating” the situation by tweeting that there would be no impunity for the attackers.

Three of the young defendants have been in pre-trial prison for over a year under a historic provision for Eta prisoners known as Fies. This means they are currently held nearly 500km from their families, whose visiting rights are restricted. Isabel Pozueta calls this a “double punishment”.

When she does see her son, she talks to him through glass by telephone. The worst, she says, was when he was given a month’s solitary confinement after protesters demanding his release marched past his prison and he shouted out to them from behind his bars.

The harsh treatment has awakened bitter memories among older residents of the days when Basques felt persecuted for their political and cultural beliefs. They fear that a fresh injustice is being committed, risking the reignition of tensions with Madrid.

Eta killed more than 850 people including civil guards, academics, and politicians in its 40-year campaign to carve out an independent state in Spain and France. It was formed in 1959 as a resistance movement to the Franco dictatorship. In Franco’s day, the Basque language, Euskara, and culture were suppressed.

Consuelo Ordóñez is the president of the victims association, Covite, which called for the Alsasua defendants to be tried for terrorism. Her brother was killed by Eta in 1995, and she fled the Basque country herself after Molotov cocktails were thrown at her home.

“It was not a casual act,” she says of the Alsasua brawl, adding that “there is more hatred towards Spain’s security forces in the Basque country and Navarre than ever before”.

Lydia Vincent, executive director of Right International Spain, says “pressure exerted on judges by attacks on their professional competence by politicians has become common”. This is especially true, she says, in sensitive cases involving protests, corruption or terrorism.

Since 2015, when Spain broadened its legal definition of terrorism in response to the jihadist threat in Europe, a total of 66 people have been convicted under the new provisions, according to Amnesty International.

They have included musicians, film-makers and journalists jailed or fined for their lyrics or posts on social media.

Family members fear that the Alsasua defendants will also be punished disproportionately.

“This is a peculiar trial,” says Edurne Goikoetxea, mother to the only girl among the defendants. “It scares us because they have not even accepted much of the evidence or included many of the witnesses.”

• This article was amended on 16 April 2018 to correct the reference to the Basque language, Euskara, and on 23 April to clarify that Euskadi is the name of the Basque autonomous community within Spain. Euskal Herria describes the entire Basque country, which comprises four provinces in Spain and three provinces in France.