The Zimbabwe Election Support Network said on Sunday the call by Mugabe for full elections in the last week of March doesn't allow enough time to establish conditions for a free vote.
The group said it is "adamant that logistically it is impossible" to meet Mugabe's timetable and complete constitutional and electoral reforms demanded by regional leaders.
It cited disputes in finalising a new Constitution, continuing political intimidation and gross inaccuracies in voters' lists that still name "ghost" electors who have long been dead.
Rushed voting couldn't be held on "a fair playing field" and the outcome would be unacceptable by democratic standards, the group said.
Mugabe has also called for a referendum on the 150-page draft Constitution in November but a parliamentary panel in charge of compiling the draft says it must be put to a stakeholders' conference first.
That conference of political parties and civic and interest groups has already been postponed to late October.
The election monitoring group said in a statement on Sunday the electoral commission responsible for running any poll has not yet received adequate funding, the draft Constitution has not been widely publicised and laws governing the referendum itself are out of date and need revision.
Prime Minister Morgan Tsvangirai, the former opposition leader in a fragile power-sharing coalition with Mugabe that was formed by regional mediators after the last violent and disputed polls in 2008, has called on his supporters to back the reformed Constitution with a 'Yes' vote in the referendum.
The regional leaders, led by the chief mediator on Zimbabwe, President Jacob Zuma, have proposed June as a more realistic time for fresh elections to end the coalition.
The poll monitoring group said an audit it carried out of voters' registration lists last year showed 27% of entries were dead voters and 47% of some 5-million voters no longer lived at the residential addresses given, opening the way for abuse and rigging.
It said sweeping security laws and media curbs favoring Mugabe have not been repealed, "hate speech" against opponents of Mugabe (88) carried by his loyalist state media has persisted and Tsvangirai's party has been denied fair access to the state broadcaster, the sole source of information for most rural Zimbabweans.
Tsvangirai (60) has been mauled in Mugabe's media over reports he had several love affairs after his wife of 30 years died in a car crash in 2009.
The state Sunday Mail newspaper reported that Tsvangirai made what it called "a stunning climbdown" at the 13th anniversary of the foundation of his party on Saturday when he apologised to "a coterie of women he inconvenienced by his escapades".
"I would like to apologise to anyone who was hurt because it was not my intention, it was a genuine search for a new wife," the paper quoted Tsvangirai saying at anniversary celebrations in the second city of Bulawayo. – Sapa-AP.