Beyene Petros has called for talks with the ruling EPRDF
An Ethiopian opposition group has called for a government of national unity after disputed elections.
The United Ethiopian Democratic Forces (UEDF) bloc made the call as initial results showed that it, and the main opposition Coalition for Unity and Democracy (CUD), had made big gains.
The opposition and the ruling Ethiopian People's Revolutionary Democratic Front (EPRDF) have both claimed victory.
The final results of Sunday's vote are due to be announced on 8 June.
The first results showed a landslide victory for the CUD in the capital, Addis Ababa, with a number of government ministers losing their seats.
The smaller UEDF also says it has captured many seats.
The ruling EPRDF has accepted that the opposition has made huge gains - winning at least a third of all seats.
But both sides say the other is guilty of major poll fraud, and both say they have won.
The National Electoral Board said it would re-run elections in six constituencies on Sunday, but the opposition parties are demanding re-runs and recounts on a much larger scale.
Dr Beyene Petros, deputy chairman of the UEDF - a coalition of 15 opposition parties - said the EPRDF may not have the number of votes necessary to form the next government.
"Therefore there is a need to have dialogue and talk between us on how to form a government of national unity," he told the BBC.
The BBC's Mohammed Adow in Addis Ababa said the UEDF's call was a little premature as the extent of the opposition's gains nationally were not yet known.
In the last parliament, the CUD and UEDF held only 12 of the country's 547 seats between them.
Turnout at the polls was higher than in previous elections
Information Minister Simon Bereket said the ruling EPRDF had won more than 300 seats.
He told the BBC that the opposition appeared to have taken around a third of the seats.
These are Ethiopia's third multi-party elections and there was massive interest, with campaigning far more open than in previous polls.
The EPRDF has governed the country since 1991.