Grand Ayatollah Hossein Ali Montazeri and Grand Ayatollah Yusef Saanei also voiced their pessimism over the prospect for a free and fair poll on 17 June.
"My point of view - and I cannot say more than this - is that things are not going in the right direction," said Montazeri, who is in his mid 80s and is one of the Islamic republic's most prominent dissidents.
"At the beginning of the revolution, the late Imam (Ayatollah Ruhollah Khomeini) and I gave promises of liberty, and these promises have not been lived up to," he said in a rare interview at his home in Qom, Iran's clerical capital just south of Tehran.
Fall from grace
Once tapped as the successor to revolutionary leader Khomeini, Montazeri says he fell from grace after he became too openly critical of political and cultural restrictions.
In January 2003 he was freed from five years of house arrest on health grounds, but his activities are still subject to tight controls.
"At the beginning of the revolution, the late Imam (Ayatollah Ruhollah Khomeini) and I gave promises of liberty, and these promises have not been lived up to"
Grand Ayatollah Hossein Ali Montazeri, Iranian Shia cleric
"I am no longer under house arrest but the way they are treating me is not correct," he said. "My offices in Mashhad and in Isfahan have been closed by the special clerics court. I am only able to give small lectures in my home twice a week."
Second critical voice
In a neighbouring street, Grand Ayatollah Yusef Saanei - a prominent pro-reform cleric and one of around a dozen senior clerics in Qom - also had harsh words for elements in the government.
"We cannot foresee the future. We do not know if we can trust the candidates to deliver on their promises and to what extent the rights of the people will be preserved and how much choice they will have," he said.
The issue of choice, he says, has emerged as a contentious issue in Iranian elections, with the Guardian Council - an unelected political watchdog - using its power to screen all candidates for public office.
Ahead of the February 2004 parliament elections, the council disqualified thousands of candidates, most of them political moderates, handing certain victory to religious hardliners.
"There should not be guardianship. In an election guardians are not needed, it is contrary to human liberty," declared Saanei, who is in his late 70s and was also one of the earliest followers of Khomenei.
Saanei, who shares the reservations of many clerics in Qom over the often tricky mix of religion and the micromanagement of the country, now largely keeps out of politics.
"There should not be guardianship. In an election guardians are not needed, it is contrary to human liberty"
Grand Ayatollah Yusef Saanei,
Iranian Shia cleric
But he has also stated that women have the undeniable right to hold the most senior positions in the country - including president or judge - even though any women seeking to stand in the 17 June election are certain to be disqualified on the grounds of their gender.
Saanei said the figure Iran needed as a future president was someone "who can follow the trend of the way that Moussavi carried out politics".
Mir Hossein Moussavi served in the now-defunct post of prime minister from 1981 to 1989, and during that period enjoyed almost constant support from Khomeini - the founding father of the regime, who died in 1989.
Reformists loyal to incumbent President Mohammad Khatami, who is at the end of his second consecutive and final term in office, have been trying to convince Moussavi - seen as a political moderate - to stand again.
But he has refused to pick up the torch of the struggling movement, leaving hardliners and conservatives dominating the race for the government's number two job.