The National Electoral Board of Ethiopia (NEBE) on Tuesday accused opposition parties of trying to undermine electoral investigation process, saying there was " nothing illegal" in the process.
NEBE deputy head Tesfaye Mengesha told a press conference the board had been conducting the investigation in accordance with the rules and regulations agreed upon by the parties.
"There is nothing that has been done in disregard of the rules and regulations," he said.
"The parties are trying to discredit the board because they are not satisfied by the decisions passed in the investigation," he added.
NEBE is responsible for the administration, conduct and supervision of all elections in Ethiopia's federal and state constituencies.
Earlier Tuesday, the Coalition for Unity and Democracy (CUD) and the United Ethiopian Democratic Forces (UEDF), the country's two major opposition parties, said in a joint statement that NEBE had rigged from the investigations into irregularities in 139 of the 524 seats contested in the May 15 parliamentary polls.
"During the investigations, there has been nothing but naked biased decisions on the part of NEBE in all of the investigating panels," the statement said, adding that "some representatives of UEDF and CUD have been forced to discontinue their involvement in the investigation."
In constituencies where the ruling Ethiopian People's Revolutionary Democratic Front (EPRDF) had lodged complaints, the statement said, more than 90 percent of the decisions are in favor of it, whereas decisions in favor of oppositions did not exceed six percent.
The statement went on to say CUD and UEDF representatives and witnesses have been "harassed," "threatened," "barred" and even " killed" upon their returning from the hearings.
With regard to the allegation, Tesfaye said it has not been substantiated by evidence.
He added that in the effort to verify the allegation by the investigation panel in the locality, it was learnt that the parties could not even identify the allegedly "killed" person to international observers.
Tesfaye said the board will continue to investigate the allegation in detail in the days ahead.
NEBE started the investigations after police shot dead at least 36 people during a protest against alleged electoral fraud.
On July 8, partial official results, released by NEBE, showed the ruling party and its smaller political allies won 158 seats against 148 seats for the opposition coalitions in the 547-seat lower house of parliament.
Results for the remaining seats will be released Friday after the electoral body finishes investigations.
Ethiopians went to poll on May 15 in the country's two separate elections to elect representatives to the House of People's Representatives and eight regional councils. Elections for the 23 constituencies of the Somali state will take place on Aug. 21.
Ethiopia has a two-house parliament: the 110-seat upper House of the Federation and the 547-seat lower House of People's Representatives.