Ethiopia has “ruthlessly targeted” and tortured thousands of people belonging to its largest ethnic group for perceived opposition to the government, rights group Amnesty International said in a report released Tuesday.

The report, based on over 200 testimonies, said at least 5,000 members of the Oromo ethnic group, which has a distinct language and accounts for over 30 percent of the country’s population, had been arrested between 2011 and 2014 for their “actual or suspected peaceful opposition to the government.”

“The Ethiopian government’s relentless crackdown on real or imagined dissent among the Oromo is sweeping in its scale and often shocking in its brutality,” said Amnesty International researcher Claire Beston.

The rights group said those arrested included students and civil servants. They were detained based on their expression of cultural heritage such as wearing clothes in colors considered to be symbols of Oromo resistance – red and green – or alleged chanting of political slogans.

Oromia, the largest state in Ethiopia, has long had a difficult relationship with the central government in Addis Ababa. A movement has been growing there for independence. And the government has outlawed a secessionist group, the Oromo Liberation Front (OLF), which has fought for self-determination for over 40 years.

Since 1992, the OLF has waged a low-level armed struggle against the Ethiopian government, which has accused the group of carrying out a series of bombings throughout the country.

Amnesty said that the majority of Oromo people targeted are accused of supporting the OLF, but that the “allegation is frequently unproven” and that it is “merely a pretext to silence critical voices and justify repression.”

“The report tends to confirm the claims that diaspora-based Oromo activists have been making for some time now,” Michael Woldemariam, a professor of international relations and political science at Boston University, told Al Jazeera. “What it does do, however, is provide a wealth of detail and empirical material that lends credibility to claims we have heard before.”

Former detainees – who fled the country and were interviewed by Amnesty in neighboring Kenya, Somaliland and Uganda – described torture, “including beatings, electric shocks, mock execution, burning with heated metal or molten plastic, and rape, including gang rape,” Amnesty said.

Although the majority of former detainees interviewed said they never went to court, many alleged they were tortured to extract a confession.

“We interviewed former detainees with missing fingers, ears and teeth, damaged eyes and scars on every part of their body due to beating, burning and stabbing – all of which they said were the result of torture,” said Beston.

Redwan Hussein, Ethiopia’s government spokesman, “categorically denied” the report’s findings. He accused Amnesty of having an ulterior agenda and of repeating old allegations.

“It (Amnesty) has been hell-bent on tarnishing Ethiopia’s image again and again,” he told Agence France-Press.

The report also documented protests that erupted in April and May over a plan to expand the capital Addis Abba into Oromia territory. It said that protests were met with “unnecessary and excessive force,” which included “firing live ammunition on peaceful protestors” and “beating hundreds of peaceful protesters and bystanders,” resulting in “dozens of deaths and scores of injuries.”

Oromo singers, writers and poets have been arrested for allegedly criticizing the government or inciting people through their work. Amnesty said they, along with student groups, protesters and people promoting Oromo culture, are treated with hostility because of their “perceived potential to act as a conduit or catalyst for further dissent.”

Al Jazeera and wire services. Philip J. Victor contributed to this report.