The members, from South Africa's opposition Democratic Alliance (DA) said they had refused to sign the preliminary report by the Southern African Development Community observer mission.
The DA said the SADC report had concluded that "despite a number of concerns, the elections were a peaceful and credible expression of the will of the people of Zimbabwe".
DA parliamentarian and mission member Diane Kohler Barnard said in a statement: "It is impossible for this deeply flawed electoral process to be viewed as a credible expression of the will of the people of Zimbabwe"
SADC, which critics say has been too soft on President Robert Mugabe, has unsuccessfully tried to mediate an end to Zimbabwe's crisis, which has left the economy in ruins and much of the population in misery.
The DA statement said the SADC mission had noted with concern threats by Zimbabwe's security forces to refuse to accept an opposition victory, the use of state resources for political purposes and the presence of police in polling stations.
The DA said the organisation of the elections was "chaotic".
More than 24 hours after the polls closed in presidential, parliamentary and local elections seen as the most crucial vote since independence, only a trickle of results had emerged on Sunday.
The DA said this fuelled fears that Mugabe's government was rigging the vote.
Zimbabwe's opposition said earlier on Sunday it had won the election but the government warned it that premature victory claims would be seen as an attempted coup.
Tendai Biti, secretary general of the main MDC opposition party, told diplomats and observers that early results showed it was victorious. "We have won this election," he said.
Projections from 12 percent of the vote showed MDC leader Morgan Tsvangirai winning 67 percent nationally, Biti said.
Results usually begin emerging soon after polls close.
Officials said the delay was caused by the complexity of counting in three different polls but Biti expressed concern.
"We're aware the results are final in most constituencies but they are deliberately taking their time to announce ... The whole idea of having an election is so you can have a result," he said.
George Chiweshe, chairman of the Zimbabwe Electoral Commission (ZEC), had to be rescued by security men in a Harare hotel when he was confronted by journalists and opposition supporters demanding results be published.
Mugabe, in power since independence from Britain in 1980, faced his most formidable challenge in the election against Tsvangirai and ruling ZANU-PF party defector Simba Makoni, who both campaigned on the collapse of Zimbabwe's economy.
Although the odds seem stacked against Mugabe, 84, analysts believe he will be declared the winner and the opposition accused him of widespread vote-rigging.
Zimbabwe's security forces, which have thrown their backing firmly behind Mugabe, said before the election they would not allow a victory declaration before counting was complete.
Government spokesman George Charamba warned the opposition against such claims. "It is called a coup d'etat and we all know how coups are handled," he told the state-owned Sunday Mail.
Residents in the eastern opposition stronghold of Manicaland said riot police stopped a victory demonstration by about 200 MDC supporters on Sunday. There was no violence, they said.
The United States said it was worried by the conduct of the election and the absence of most international observers.
"The Mugabe regime is a disgrace to the people of Zimbabwe and a disgrace to southern Africa and to the continent of Africa as a whole," Secretary of State Condoleezza Rice told reporters during a visit to Jerusalem.
Once-prosperous Zimbabwe is suffering from the world's highest inflation rate of more than 100,000 percent, chronic shortages of food and fuel, and an HIV/AIDS epidemic that has contributed to a steep decline in life expectancy.
Mugabe accuses the West of sabotaging Zimbabwe's economy. He rejected vote-rigging allegations.