The main opposition coalitions and the ruling Ethiopian People's Revolutionary Democratic Front of Prime Minister Meles Zenawi claimed victory in May 15 elections to the 547-seat federal parliament.
Official results are not expected until June 8.
The Coalition for Unity and Democracy, or CUD, said its evidence of voting irregularities must be investigated by a neutral body made up of international observers, some from the CUD and the ruling party, and the Ethiopian Election Board.
"At present, we can see clearly that the free and fair election process is in real danger. On our part we have compiled evidence that election irregularities have been observed in 139 constituencies," CUD Chairman Hailu Shawul told a news conference.
Unless the investigations and re-votes are held, "our party will not join the next parliament," he said.
Last week, the party demanded repeat polls in 27 districts, and a vote recount in 57 others.
The election board held re-votes in six constituencies Saturday and said it was looking into other charges of impropriety in an unspecified number of districts.
Hailu warned that people may resort to civil disobedience such as sit-in strikes, absenteeism and prayer vigils if the ruling party goes ahead and forms the next government.
Ethiopian Information Minister Bereket Simon reiterated the ruling party's promise to clamp down on post-election violence or disobedience.
"Nothing can hold back the [Ethiopian People's Revolutionary Democratic Front] from forming the next government," he said. "It should be known to all that defying the verdict of the people will not be accepted."
Photo Courtesy: AFP/File/Marco Longari
Coalition for Unity and Democracy supporters. Ethiopia's opposition threatened to boycott the next session of parliament unless its complaints of widespread vote fraud in last week's elections are resolved(AFP/File/Marco Longari)