Former President Ali Akbar Rafsanjani of Iran has called for the release of people jailed after protesting at the result of the election.
In his first Friday sermon since the vote on 12 June, he said large numbers of Iranian people still doubt its result.
Meanwhile, pictures and videos from Iran posted on the internet show thousands of opposition supporters chanting anti-government slogans after Friday prayers in the capital Tehran.
It was the largest street protest in weeks.
Police fired tear gas to disperse the crowd and there are are reports of several arrests, including a well-known women's rights activist, Shadi Sadr.
Iran's state media did not report the clashes.
There were chants of support for defeated election candidate Mir Hossein Mousavi, who attended the prayers.
Mr Mousavi, who attended the Friday prayers at which Mr Rafsanjani spoke, has demanded a re-run of the vote and described the new government as illegitimate.
Another defeated opposition candidate, Mehdi Karoubi, also attended the prayers, according to the website of his party, Etemad Melli.
The BBC reports Friday's rally followed warnings from a minister against turning the occasion into a "stage for undesirable scenes".
The post-election protests are seen as the worst crisis in the Islamic republic since the revolution in 1979.
New nuclear chief
The new head of Iran's Atomic Energy Organisation said on Saturday the Islamic state and the West needed to make more efforts to gain mutual trust to end the dispute over Tehran's nuclear programme.
It was Ali Akbar Salehi's first official comment since he took over from Gholam Reza Aghazadeh, who has resigned following 12 years as head of the organisation.
The ISNA news agency says Mr Aghazadeh submitted a letter of resignation to President Mahmoud Ahmadinejad nearly three weeks ago. It was not immediately clear what triggered the resignation.
Iran says its nuclear programme is for civilian power, denying Western claims that it wants a nuclear bomb.
ISNA reports Mr Aghazadeh has also stepped down as Iran's vice-president.
The agency says he has given no explanation for his resignation, which has been accepted by Mr Ahmadinejad.
Mr Aghazadeh is a veteran official who served in the 1980s as a deputy to Mir Hossein Mousavi - the defeated candidate in Iran's disputed presidential elections last month.
He was appointed oil minister in 1985, staying in the post for 12 years during the presidency of Ali Akbar Hashemi Rafsanjani.
He then moved to his job at the head of the atomic agency in 1997 under former president, Mohammad Khatami.
He continued in the post when Mr Ahmadinejad was first elected in 2005.
The BBC reports it is not clear whether Mr Aghazadeh's resignation was linked to the disputed presidential elections on 12 June.
Control over Iran's foreign and nuclear policies ultimately lies with Supreme Leader Ayatollah Ali Khamenei.