Official results from last month's parliamentary elections in Ethiopia will be delayed by one month, and are now due to be announced on 8 July.
This will allow election authorities to investigate hundreds of alleged episodes of fraud and vote rigging.
Opposition parties have vowed to stage mass protests if the provisional results are validated.
EU observers have voiced concern over irregular vote counting and biased reports by the state-owned media.
According to provisional results, the ruling Ethiopian People's Revolutionary Democratic Front (EPRDF) and its allies have won an overall majority of 320 seats in the 547-member parliament.
The main ruling party, which has been in power for 14 years along with its allies, has acknowledged it has lost votes to the opposition, but has won enough seats to form a new government.
But the opposition has accused the ruling coalition of irregularities and intimidation.
The two main opposition groups have also filed a lawsuit against the National Electoral Board of Ethiopia (NEBE), accusing it of conspiring with the government to rig the polls.
The board in its turn has announced it will sue them for defamation.
"Such statements, though baseless and fabricated, may mislead the public and discredit the efforts of the NEBE," the board said in a written statement.
Fresh ballots have been held for at least six disputed seats.
In a separate development, six journalists working for independent newspapers in the capital are said to have been detained and questioned over their coverage of the elections.