"The credibility of the 2013 Harmonised Elections is seriously compromised by a systematic effort to disenfranchise an estimated million Voters," it said in a statement, adding that "the voter registration process was systematically biased against urban voters."
ZESN said that potential voters were turned away from 82 percent of urban polling stations - but in rural areas, seen as strongholds of President Robert Mugabe, it said, the percentage was less than half that.
Mr Mugabe's allies are claiming a victory in Wednesday's poll.
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