Ethiopian police investigators in Addis Ababa's main detention centre have tortured political detainees and regularly mistreat people in custody to extract confessions, Human Rights Watch (HRW) said on Friday.
The Ethiopian government, long seen by the West as a bulwark against militant Islam in the Horn of Africa, has denied frequent accusations that it uses state institutions to stifle dissent and silence political opposition.
It dismissed the HRW report on Friday as "trash".
In its report on conditions inside Addis Ababa's Federal Police Crime Investigation Sector, known as Maekelawi, HRW said many former detainees were slapped, kicked and beaten with sticks and gun butts during investigations.
"Human Rights Watch found that investigators used coercive methods, including beatings and threats of violence, to compel detainees to sign statements and confessions," the group said in a statement, referring to events over the past three years.
Ethiopia intensified its clampdown on peaceful dissent after the disputed 2005 election, the New York-based HRW said.
Human Rights Watch said scores of opposition politicians, journalists, protest organisers and alleged supporters of ethnic insurgencies have been detained in Maekelawi.
Interviews with more than 35 former detainees and their relatives formed the basis of the report, HRW said.
Government spokesman Shimeles Kemal rejected the findings.
"The report is marked by excessive reliance on questionable and unverifiable testimonies. It also exhibits clear omission of specific facts and evidences contrary to international standards of monitoring," Kemal told Reuters.
"Human Rights Watch... does not have a presence in the country and as a result it lacks the capacity to produce balanced and objective reporting. All in all it is trash."
Some Muslims have complained that the government interferes in religious matters as it tries to stop what officials say is a rise in Islamist ideology. Ethiopia has a Christian majority but about a third of its population is Muslim.
Last week, Ethiopian Prime Minister Hailemariam Desalegn dismissed suggestions that his government meddles in religion.