Ethiopia has “ruthlessly targeted” and tortured its largest ethnic group owing to a perceived opposition to the government, Amnesty International has said.

Thousands of people from the Oromo ethnic group have been “regularly subjected to arbitrary arrest, prolonged detention without charge, enforced disappearance, repeated torture and unlawful state killings,” according to a damning report based on more than 200 testimonies. “Dozens of actual or suspected dissenters have been killed.”

At least 5,000 Oromos have been arrested since 2011 often for the “most tenuous of reasons”, for their opposition – real or simply assumed – to the government, the report added.

Many are accused of supporting the rebel Oromo Liberation Front (OLF).

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

One young girl said hot coals had been dropped on her stomach because her father was suspected of supporting the OLF, while a teacher described how he was stabbed in the eye with a bayonet after he refused to teach “propaganda about the ruling party” to students.

There was no immediate response from the government, which has previously dismissed such reports and denied any accusation of torture or arbitrary arrests.

“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,” the Amnesty researcher Claire Beston said.

“This is apparently intended to warn, control or silence all signs of ‘political disobedience’ in the region,” she added, describing how those she interviewed bore the signs of torture, including scars and burns, as well as missing fingers, ears and teeth.

With nearly 27 million people, Oromia is the most populated of the country’s federal states and has its own language, Oromo, which is distinct from Ethiopia’s official Amharic language.

Some of those who spoke to Amnesty said people had been arrested for organising a student cultural group. Another said she was arrested because she delivered the baby of the wife of a suspected OLF member.

“Frequently, it’s because they refused to join the ruling party,” Beston added, warning that many were fearful attacks would increase before general elections slated for May 2015.

In April and May, security forces shot dead student protesters in Oromia. At the time, the government said eight had been killed, but groups including Human Rights Watch said the toll was believed to be far higher. Amnesty said “dozens” had been killed in the protests.