The Horn of Africa country has won international plaudits for delivering double-digit growth for much of the past decade, but rights groups often accuse the government of using state institutions to stifle dissent and silence political opposition.
Addis Ababa, long seen by the West as a bulwark against militant Islam in the Horn of Africa, denies charges that it is quashing dissent.
In a 39-page report launched on Thursday, the Unity for Democracy and Justice (UDJ) detailed what it said were "gruesome rights violations" committed against its supporters and members.
"One hundred and fifty members and supporters of the party have been subject to severe beatings, illegal detentions and abductions by police and security officials," party chairman Negasso Gidada told reporters.
"We are asking the government to stop these human rights violations and take those responsible to justice," said Negasso, who served as the country's president from 1995 to 2001, before joining the opposition.
A government spokesman declined to comment saying it had to receive the report.
Earlier this month, Human Rights Watch said many former detainees - including politicians, journalists and alleged supporters of insurgencies - were slapped, kicked and beaten with sticks and gun butts during investigations at Addis Ababa's Federal Police Crime Investigation Sector, known as Maekalawi.
Ethiopia intensified its clampdown on peaceful dissent after the disputed 2005 election, the New York-based watchdog said.
Back then, the disputed polls ended in violence and the killing of 200 people. Opposition candidates won 174 seats but many did not take them up, saying the vote was rigged.
In an interview with Reuters this month, Prime Minister Hailemariam Desalegn said the government was not to blame for the opposition's poor showing.
He has also accused some opposition party members of collaborating with rebel groups the government had previously labelled as terrorist organisations.
But UDJ's leaders deny any links with the outlawed rebel groups, and warn the government that "stifling" dissent may encourage violence in the country.
"We are not requesting anything from the government side, we are requesting a level playing ground," Girma Seifu, a senior UDJ official and the sole opposition politician in Ethiopia's 547-seat parliament.