Commonwealth observers say Robert Mugabe's re-election was held in a climate of fear and did not reflect the will of Zimbabwe's people.
The group's preliminary report lists a series of violations including the activities of paramilitary youth groups.
Their condemnation comes on top of concern expressed in London and Washington, where President George Bush denounced the outcome as flawed and said the US did not recognise it.
Mr Mugabe got 54% of the vote compared with opposition leader Morgan Tsvangirai's 40%.
The Commonwealth observers' report expressed concerns about supporters of Mr Mugabe's ruling ZANU-PF party.
"All of the foregoing brings us to the conclusion that there was not a free expression of will by the electorate," it said.
Thousands of Zimbabweans were disenfranchised and a systematic campaign of intimidation against opposition supporters "created a climate of fear and suspicion", said the report.