A senior pro-reform cleric said many Iranians remained unconvinced about President Mahmoud Ahmadinejad's re-election because of voting "ambiguities" and the government could face problems, an Internet statement said.
Grand Ayatollah Yusof Saanei also urged the authorities not to violate people's rights, in an apparent reference to their handling of mass protests that erupted after last month's disputed presidential election.
"I remind you that no instruction or command can be a permission or excuse to violate people's rights and this could be a great sin," he said in a statement posted on his website on Friday.
The authorities reject opposition charges of vote rigging, saying the election was the country's "healthiest" since the 1979 Islamic revolution that toppled the U.S.-backed shah.
They have portrayed the protests as the work of local subversives and foreign powers, especially Britain.
Saanei is an ally of Grand Ayatollah Hossein Ali Montazeri, Iran's most senior dissident cleric. Montazeri was an architect of the Islamic revolution who fell out with the present leadership and was under house arrest for some years.
The June 12 vote stirred the most striking display of internal dissent in Iran since the revolution three decades ago and strained ties with the West.
The authorities have blamed defeated candidate Mirhossein Mousavi, a moderate former prime minister, for post-election violence, in which at least 20 people were killed.
Mousavi, who says the poll was rigged in favour of the hardline incumbent, rejects the charge.
"I hope that the path of the Iranian people to continue their legal protest could be open and that particularly the youth ... pursue their activities calmly, peacefully and successfully," Saanei said.
Although hardliners have appeared to be in the driving seat since security forces overcame street protests that erupted in the days after the poll, Mousavi and another losing candidate, pro-reform cleric Mehdi Karoubi, have not yielded.
They again denounced the election result on Wednesday and said Ahmadinejad's next cabinet would be illegitimate.
"This means that they and many of the people have not yet been convinced over the ambiguities in the election ... and it is possible that (the government) faces legal and civil problems," Saanei said.
Despite the defiance, analysts say the defeated candidates have few practical options after Iran's top legislative body on June 29 certified results showing Ahmadinejad had won re-election by a landslide.
The conservative clerical leadership is likely to ignore the reformers and keep backing Ahmadinejad, who enjoys the public support of Supreme Leader Ayatollah Ali Khamenei, Iran's top authority.
(Writing by Fredrik Dahl; Editing by Charles Dick)