Mousavi says he is responding to calls to protect people's rights
Iran's defeated opposition presidential candidate, Mir Hossein Mousavi, says he will make his first public appearance for several weeks at Friday prayers.
In a message to supporters on his web site, Mr Mousavi said he would attend congregational prayers at Tehran University as a show of solidarity.
The sermon is due to be delivered by former president and critic of the re-elected president, Hashemi Rafsanjani.
Mr Mousavi's supporters bitterly dispute the result of the 12 June poll.
"I will join the lines 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," Mr Mousavi said on Ghalamnews website.
Another defeated candidate Mehdi Karroubi said he would join the congregation at what will be Mr Rafsanjani's first sermon in more than two months, reports say.
Massive street protests followed the presidential election in which at least 20 people were killed in violence and hundreds were arrested, according to official figures.
Mr Mousavi vowed on Wednesday not to let the blood of protesters killed by security forces and pro-government militias be spilt in vain, a reformist website reported.
He was speaking during a visit to the family of Sohrab Arabi, a teenager killed in protests.
The country's most senior political figure, Supreme Leader Ayatollah Ali Khamenei, upheld Mahmoud Ahmadinejad's landslide victory and demanded an end to protest.
Correspondents say it is not clear whether Mr Mousavi's supporters will manage to gather in large numbers around the university in Enghelab Square on Friday, where some of the largest post-election demonstrations took place.
Pro-government media have been warning about the possibility of "provocation" at Friday prayers.
One newspaper called on worshippers "not to be deceived and reject those who shout divisive slogans".