Election observers have been arrested across Ethiopia on the eve of Sunday's poll, opposition members say.
The campaign has been marked by an unusually lively media debate
They told a gathering in the capital, Addis Ababa, that more than 100 monitors remained in police custody. One candidate is also being held.
The Ethiopian Human Rights Council also rejected assessments that the election was largely peaceful so far.
The poll is widely seen as a test of the Ethiopian government's willingness to bring democracy to the country.
The election is the third under Prime Minister Meles Zenawi and his ruling Ethiopian People's Revolutionary Democratic Front (EPRDF) party.
The campaign has been marked by unprecedented openness, with rallies by both the government and opposition attracting hundred-of-thousands of supporters and both sides being given wide access to the media.
But human rights groups and opposition parties say there has still been widespread intimidation.
"Even on the eve of the voting, our party observers are being arrested and denied access," Beyene Petros, vice-chairman of United Ethiopian Democratic Forces (UEDF), told the BBC.
Mr Petros said that in Konteb, in the Hadiya zone, 230km (143 miles) south of Addis Ababa, 38 observers and one election candidate had been detained.
"We are extremely distressed, having worked very hard... The reports we are receiving are only the tip of the iceberg," Mr Petros said at a later press conference.
Opposition leaders have rejected former US President Jimmy Carter's positive assessment of the nature of campaigning.
Andargatchew Tesfaye, head of the Ethiopian Human Rights Council, said about 300 international observers were not enough to make a proper assessment.
Mr Tesfaye said he was unsure how many of the group's 1,644 observers would be in place when polls open on Sunday.
He said that a legal challenge to the group's right to post observers, launched by the National Electoral Board, was a deliberate delay tactic designed to prevent independent observers from reaching rural areas in time.
Speaking to the Associated Press news agency, government spokesman Zemedkun Tekle said criticism from the Ethiopian Human Rights Council was entirely expected.
"The government has repeatedly made it clear this organisation is not neutral and has already decided its verdict on the elections before it is held," Mr Zemedkun said.