The US has rejected Zimbabwean President Robert Mugabe's election win and the European Union has expressed concern over the poll in contrast with African states, which have broadly endorsed the vote.
US Secretary of State John Kerry has said the results are the culmination of a deeply flawed process and do not represent the will of the country's citizens.
EU foreign policy chief Caroline Ashton said the bloc was concerned about irregularities, alleging a lack of transparency.
Mr Mugabe was declared the winner with over 61% of the votes, versus nearly 34% for his rival Morgan Tsvangirai, Movement for Democratic Change (MDC) leader and prime minister.
Observers from the African Union and Southern African Development Community (SADC) which monitored the elections broadly endorsed them as free and peaceful, while acknowledging some minor problems.
Independent domestic monitors have however described the vote as deeply flawed by registration problems that may have disenfranchised up to a million people.
Mr Tsvangirai has called on the African Union and SADC to investigate the vote, calling it "null and void" and "not credible".
"We are going to go to court, we are going to go to the African Union, we are going to go to the SADC," Mr Tsvangirai angrily told a news conference in Harare. He rejected the result as "fraudulent".
Mr Kerry called on both organisations to address their concerns with the election.
"In light of substantial electoral irregularities reported by domestic and regional observers, the United States does not believe that the results announced today represent a credible expression of the will of the Zimbabwean people," Mr Kerry said in a statement.
"There were irregularities in the provision and composition of the voters roll. The parties had unequal access to state media. The security sector did not safeguard the electoral process on an even-handed basis," he added, also calling on all parties to refrain from violence.
The European Union also criticised the vote, from which Western election observers were barred by Harare.
"The EU is concerned about alleged irregularities and reports of incomplete participation, as well as the identified weaknesses in the electoral process and a lack of transparency," EU foreign policy chief Catherine Ashton said.
Western rejection of the regional African verdict on the election could stir tensions with the continent but acceptance of Mr Mugabe's win will be criticised by those who say he is a despot guilty of rights abuses and ruining the economy.