By Jay Deshmukh
Rafsanjani, a target of intense mud-slinging by President Mahmoud Ahmadinejad during the June presidential election campaign, will be leading the prayers for the first time in more than two months.
Ahmadinejad's main challenger Mir Hossein Mousavi is also expected to attend the prayers, marking his first public appearance in weeks.
Witnesses said crowds of people were heading towards Tehran university, the venue for the prayers, as dozens of policemen and Islamist vigilantes took up positions on streets leading to the campus.
"People are coming in groups, walking slowly and silently. They are not chanting any slogans," a witness said, adding that police had banned vehicles from some nearby streets.
Thousands of Mousavi's supporters are expected at the venue, setting the scene for possible confrontation with regular hardline worshippers.
Iranian Intelligence Minister Gholam Hossein Mohseni Ejeie has already expressed concern over the prayers.
"Iranian people must be careful that the Friday prayers are not turned into a venue for unpleasant scenes," he said on Thursday.
It will be Mousavi's first public appearance since his supporters last month held massive street protests in Tehran against Ahmadinejad's bitterly disputed re-election.
"I will join the lines (of worshippers) on Friday as I feel obliged to respond to the call of companions on the path to protecting rights to a noble and free life," Mousavi said on his website Ghalamnews.
Mehdi Karroubi, the other defeated presidential candidate is also attending the prayers, his party newspaper said.
Mousavi has charged that the June vote was rigged and has dismissed the next government as "illegitimate".
The post-election anti-Ahmadinejad protests saw hundreds of thousands of demonstrators take to the streets of Tehran and other cities, triggering the worst crisis in the Islamic republic since the 1979 revolution.
The ensuing violence left at least 20 people dead, many scores wounded and hundreds arrested, according to official figures.
The protests shook the pillars of the Islamic republic and split the nation's clerical groups, while the crackdown by the authorities on demonstrators provoked worldwide outrage.
Iranian security forces backed by members of the volunteer Islamic Basij militia managed to stifle the protests, with all public gatherings banned, but demonstrators have defiantly taken to streets on several occasions.
As word spread of Mousavi joining the prayers, media reports said his supporters are mobilising to attend the sermon, seen as a yet another attempt to defy the authorities who have banned any kind of gatherings by the opposition groups.
The prayers have added significance as Rafsanjani, a key Mousavi supporter who himself lost out to Ahmadinejad in the 2005 presidential race, is expected to break his post-election silence.
Rafsanjani is yet to comment on election results although a key clerical institution, the Assembly of Experts, which he heads, hailed the mass turnout but made no mention of Ahmadinejad's victory in its post-vote statement.
He had come under attack from Ahmadinejad during a prime-time television debate in the run-up to the vote, with the hardline incumbent accusing Rafsanjani's family of corruption.
Iran's leading hardline newspaper Kayhan warned against "provocation" at the Friday prayers.
"We have even heard that some with a hezbollahi (Islamist) appearance intend to carry out these provocations. So worshippers should be careful not to be deceived and reject those who shout divisive slogans," it said.
Enghelab (Revolution) street, where the university is based, was scene of some of the biggest post-election protests.
Security forces brutally cracked down on protesters in the area on June 20 after Iran's supreme leader Ayatollah Ali Khamenei urged an end to protests and warned that opposition leaders would be responsible for any bloodshed.
