Mousavi's statement, posted late on Wednesday, confirmed a media report earlier this week that he would attend the prayers at Tehran University to be led by former president Akbar Hashemi Rafsanjani, a rival of re-elected President Mahmoud Ahmadinejad.
Mousavi, Ahmadinejad's main challenger in the June 12 presidential election, says the vote was rigged in the hardline incumbent's favour. The authorities reject charges of vote fraud.
"Since I regard as obligatory responding to the invitation of the sympathisers and supporters in the path of safeguarding legitimate rights of a free and honourable life, I will maintain a presence alongside you on Friday," Mousavi said.