Ethiopia's national elections failed to meet international standards for democracy in key areas, the European Union's (EU) chief observer said on Thursday.
Ana Gomes said post-polling day irregularities, delays and questionable vote counting, as well as flawed handling of complaints and re-runs of elections in some constituencies had undermined the electoral process.
The ruling party in turn accused Gomes and the EU observer mission of bias.
"The report is basically biased and clearly indicates the partisan position she and her mission have taken," said Bereket Simon, Ethiopia's information minister, who regained his seat in a re-run of the election after being narrowly defeated.
"She has betrayed bias despite the facts on the ground," he added.
The EU's 10-page election report focused on the massive complaints lodged by all political parties after the 15 May vote, and on re-runs of elections and delayed polls in eastern Ethiopia's Somali region.
"In several important aspects, international standards for genuine democratic elections were not met," Gomes told reporters and diplomats at a press conference in the capital, Addis Ababa.
The EU mission, which first arrived in Ethiopia in March, was one of the largest ever, with 160 observers. It is expected to depart at the end of the week and will publish a final election report in September when all the results are known.
Opposition parties, for their part, welcomed the report. The vice chairman of the largest opposition group, the Coalition for Unity and Democracy, Berhanu Nega, said he hoped the findings would pave the way for dialogue with the government to help break the current political impasse.
Officials from the National Electoral Board of Ethiopia (NEBE) declined to make an immediate comment.
"The NEBE is reviewing this report and will officially respond on Friday," Getahun Amogne, spokesman for the board, said.
The report comes amid increasing tensions between the EU and Prime minister Meles Zenawi's ruling Ethiopian People's Revolutionary Democratic Front (EPRDF), which won a majority of seats in parliament following a bitter election marred by widespread claims of fraud and vote rigging.
At the weekend, the EU issued a statement defending Gomes after state media accused her of bias towards opposition parties. Gomes told journalists she was considering legal action against parts of the state media for the allegations against her, describing them as "baseless garbage".
According to local human rights groups, security forces killed some 42 people when hundreds took to the streets to protest alleged election fraud. Security forces also arrested and held thousands of protestors and opposition members.
The EPRDF has so far won 296 seats in the 547-member parliament, while its allied parties won 22 seats. Opposition parties won over 160 seats. Results are still outstanding for 54 seats.