Opposition website Kaleme.Com reported seven arrests of participants in a ceremony in the Jamaran mosque in northern Tehran yesterday, with relatives saying "they were taken to Tehran's Evin prison".
Conservative website nasimonline.Ir and alef.Ir also reported the arrests and showed pictures of former premier Mousavi being held by some people at the gathering.
Mousavi and reformist Mehdi Karroubi were defeated by Mahmoud Ahmadinejad in the 2009 presidential election, and alleged fraud when the incumbent was re-elected.
Both Mousavi and Karroubi have now been under house arrest for more than two years.
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Media also reported that banners reading "Death to the dictator", "Political prisoners must released" and "The prime minister of the Imam (Ayatollah Ruhollah Khomeini) must be freed," were seen at the meeting.
Mousavi was prime minister, a now defunct post under the new constitution, from 1981 to 1989 during Khomeini's rule.
A spokesman for Rowhani's campaign did not confirm the arrests to AFP, saying that "some people tried to disrupt the meeting".
The moderate Rowhani is one of eight candidates approved by constitutional watchdog the Guardians Council to stand for president on June 14.
He is considered close to former reformist president Mohammad Khatami (1997-2005), and was Iran's chief nuclear negotiator between 2003 and 2005.
Although reformists are weak politically, Rowhani is hoping to harvest their votes along with those who would have voted for the former pragmatic president, Akbar Hashemi Rafsanjani.
Rafsanjani was disqualified from standing by the Guardians Council.