Elections in Ethiopia's biggest region will be a "hollow exercise", says lobby group Human Rights Watch.
In the capital, some feel able to show their opposition support
It says that government repression has increased in Oromia, home to 32% of Ethiopians, ahead of Sunday's poll.
The report comes after European Union election observers wrote to Ethiopia's ruling party, voicing their concerns.
They complained of intimidation of opposition officials and "hate speech" but also praised the relative lack of violence so far.
At the weekend, hundreds of thousands of people attended rallies in the capital, Addis Ababa, without incident, which one observer called a "miracle".
"The pervasive pattern of repression and abuse documented in this report ensures that voting on May 15 will be a hollow exercise for most of Oromia's population," HRW says.
Ethiopia's Information Minister Bereket Simon rejected the accusations, saying the report's findings had nothing to do with reality.
Last week, Prime Minister Meles Zenawi used a national televised address to accuse the opposition of fomenting ethnic hatred and compared them to the militias responsible for Rwanda's 1994 genocide.
Chief EU election observer Ana Gomez condemned such talk and said she wanted "action, not promises" to put things right.
She said she was concerned about beatings of opposition officials and disruption of their rallies.
"I myself spoke with people who have been beaten," she told the AFP news agency.
However, she said the overall picture was "very positive".
For many years, Ethiopia was a one-party state and thousands of government critics were killed in the "Red terror".
"Never before in Ethiopian history has there been such an open debate in the country," said the head of the EU delegation in Ethiopia, Tim Clarke.
"For people who have been here a long time, it's a miracle what is happening these days.
"Yes, there are deficiencies, [but] this is only the third election in the country."
Mr Meles was re-elected in parliamentary elections in 2000, following Ethiopia's first multi-party elections in 1995.
The European Union is sending 150 monitors to observe the elections.
Meles Zenawi has won both of Ethiopia's multi-party elections
In March, six US election observers were expelled from Ethiopia on the grounds they were operating illegally and "not invited".
Some 25 million Ethiopians are able to vote for new members of parliament, who in turn choose a prime minister.
Some 35 parties are contesting the seats, although most of these are members of the three main coalitions: the ruling Ethiopian People's Revolutionary Democratic Front (EPRDF) and the opposition CUD and UEDF.