Former Iranian President Akbar Hashemi Rafsanjani has implicitly criticised the new president for conducting a purge of government officials.
Hashemi Rafsanjani was runner-up in June's presidential election
He said Mahmoud Ahmadinejad's policies were damaging Iran and helping its enemies in a speech to a group of Friday prayer leaders on Wednesday.
Mr Rafsanjani also complained that former officials were being accused of corruption without proof.
On Monday, Iran's supreme leader called for an end to such criticism.
A veteran of the 1979 Islamic revolution, Mr Rafsanjani came second to Mr Ahmadinejad in June's presidential election.
He currently chairs the Expediency Council, an advisory body for the Supreme Leader Ayatollah Ali Khamenei which has final adjudicating power in disputes over legislation between the parliament and the Guardian Council.
Helping Iran's 'enemies'
Mr Rafsanjani began by criticising those who removed long-standing officials and accused them of corruption without proof.
"Today some are calling into question the actions of the past, and are enacting a policy of purges, a policy of general banishment and the sidelining of competent people," the state news agency Irna quoted him as saying.
"These people are muddying others, and if we let them do so, they will call into question the achievements of the regime and the revolution."
The BBC correspondent in Tehran, Frances Harrison, says this was a clear reference to President Ahmadinejad, who has talked of cutting off the hands of the corrupt.
The president has replaced a number of high-ranking officials since he was elected, including the directors of all of Iran's state-run banks and 40 of Iran's most senior diplomats.
Mr Rafsanjani warned that such policies would threaten the Islamic republic and appealed for increased national unity.
"Such attitudes will allow the enemies to reach their objectives," he said.
"Through solidarity, we must protect the diamond of our revolution; otherwise it will crack, and that will be an unforgivable crime."