DUBAI, United Arab Emirates (AP) — Bahrain has sentenced 13 prisoners to an additional three years over a sit-in held in a detention facility in 2021 that prison authorities say was violent.

The Bahrain Institute for Rights and Democracy said Wednesday that the convictions issued the day before stemmed from a mass trial of 65 defendants, the rest of whom were acquitted. It alleged “severe due process violations, including the right to attend the trial, or meeting with the lawyer.”

It also publicized what it said were firsthand accounts given to the public prosecution in which the prisoners said they were beaten with metal objects and tear-gas cannisters.

Bahrain’s prison authority denied the allegations, saying prisoners’ legal rights are guaranteed and that any allegations of mistreatment are thoroughly investigated. It said inmates taking part in the sit-in had attacked and injured guards, and damaged public property.

Bahrain launched a heavy crackdown on dissent in response to mass protests in 2011 calling for the overthrow of its monarchy. The tiny island nation in the Persian Gulf has a Shiite majority but is ruled by a Sunni monarchy that is closely allied with Saudi Arabia and the United Arab Emirates, which helped crush the revolt.

Dozens of prisoners being held in the Jaw Rehabilitation and Reform Center staged a sit-in in April 2021 after an inmate died, allegedly from being denied medical care. The Bahrain Institute says special forces and officers used “excessive force” against the prisoners and tortured many of them. Some were held incommunicado for up to 36 days, it said.

“This mass trial demonstrates a core problem in Bahrain’s corrupt judicial system, where prisoners of state violence and victims of torture are condemned while torturers avoid any accountability,” Sayed Ahmed AlWadaei, director of advocacy for the institute, said in a statement.

Bahrain’s General Directorate of Reform and Rehabilitation denied the sit-in was peaceful, calling it a “pre-meditated violent attempt by a small but well-organized group of inmates to disrupt the facility’s operations.” It said the prisoners had blocked corridors and disrupted services by “violent means.”

“The facility’s staff took proportionate measures, in line with prison policy, that were necessary to protect staff and inmates and to ensure the continued delivery of essential services, namely healthcare for inmates,” it said in a statement.

Inmates at the same facility recently held a monthlong hunger strike to protest their conditions. They suspended the strike earlier this month after prison authorities promised to limit isolation, expand visitor rights, extend the hours of daylight and improve health care.

Bahrain is a close Western ally that hosts the U.S. Navy’s 5th Fleet. It was also one of four Arab nations that normalized diplomatic relations with Israel in the so-called Abraham Accords negotiated by the Trump administration in 2020.