Speaking on BBC Newsnight, Fungayi Mabhunu accused a Zanu-PF spokesman, Nick Mangwana, of lying when he promised the country would hold open elections next year.
He said: “We don’t believe anything that comes from Zanu-PF.
“They’ve deceived us and hoodwinked us in the past. What makes you think that we believe them now?”
The anti-Mugabe activist accused the ruling party of an internal power struggle.
On Tuesday night the Zimbabwe military took control of the country’s ZBC state broadcaster and placed Mr Mugabe under house arrest.
However, the army has denied a coup d’etat, labelling the events a “bloodless takeover” of power.
Mr Mangwana, a leading Zanu-PF member living in the UK, had said that national presidential elections would be held next year as originally planned.
He promised the vote would be “free and fair” and that as a result, his party could end up in opposition.
Zanu-PF has won every presidential election since Zimbabwe became an independent country, despite accusations of electoral malpractice.
In the 2008 presidential election, Mr Mugabe was accused of tampering with the vote to guarantee he won.
The opposition party’s candidate, Morgan Tsvangirai, received the highest percentage share of the vote (47.9 per cent) but a second presidential election was held after no candidate won more than 50 per cent of the vote.
A run-off election was held and Mr Mugabe was re-elected for a seventh term.
Mr Mangwana said Mr Tsvangirai had failed to become President because he did not receive over 50 per cent of the vote.
He said: “We don’t do first past the post in Zimbabwe.
“You have to get 50 plus one so he didn’t meet the threshold.”
However, Mr Mabhunu argued that the country’s leading party had manipulated elections in the past and the public would only believe in Zimbabwe democracy if neutral observers watched over the elections.
He said: “The people of Zimbabwe will only believe in Zimbabwe if we have free and fair elections that are internationally monitored.”