The fight at a bar in the small town of Alsasua in the early hours of Oct. 15, 2016, left one off-duty officer with a broken ankle and caused psychological trauma to all the victims, according to Civil Guard unions.
Prosecutors say the brawl should be punished under the country's terrorism laws, as crimes of terrorist intent. They claim the attackers were members of Basque activism groups and that targeted the off-duty officers as part of their campaign to oust Spanish security forces from Basque areas. The goal is also shared by the militant separatist group ETA.
Areas with a strong Basque identity are grappling to overcome decades of violence at the hands of ETA, which killed more than 800 people including police, politicians and entrepreneurs before giving up its armed campaign for Basque independence in 2011.
Supporters say the charges are disproportionate for what they consider that was a simple bar brawl. They also complain that three of the defendants have been kept in pre-trial custody for nearly 18 months. They also consider the prosecutors' request for prison sentences ranging between 12 and 62 years as unreasonable.
"It would be a real judicial aberration to uphold the charges," defense lawyer Manuel Olle said Monday. "That would be trivializing terrorism in the criminal code."
Hearings have been scheduled for the next two weeks in the highly anticipated trial that opened Monday at the National Court in Madrid. A verdict could be months away.
Backed by regional and local authorities, tens of thousands of people protested over the weekend in the northern city of Pamplona, the capital of the province where Alsasua is located. The protesters marched behind a banner with the slogan "Justice. It's not terrorism."
Relatives of the defendants at the march said they were not backing impunity but urged that justice be applied "without exaggeration, distortion and disproportion."
The Collective for the Victims of Terrorism, or COVITE, which is part of the prosecution in the trial, said Saturday's march and local officials' support for it was a further attack on the victims and "a clear abandonment" of them.
COVITE's lawyer, Miguel Mugica, told Cadena SER the attack on the officers should be seen as part of a "totalitarian climate that remains in the streets of the Basque Country and Navarra and that seeks to silence dissidents."
Amnesty International has said the violent aggression should be tried in a lower regional court in Navarra, and not at the National Court, which typically handles terrorism cases.
Inactive since 2011, ETA gave up on its remaining arsenal last year. Both France and Spain, where the group committed most of its deadly actions over more than 40 years, have demanded it take a further step and disband.
Copyright 2018 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.