Former presidents

Even less clear are the intentions of former presidents Akbar Hashemi Rafsanjani and Mohammad Khatami, whose moderate candidacies would radically alter the contest. Both appear reluctant to stand unless given the nod from Khamenei but neither have ruled themselves out.

"I will not enter the field without his consent," Rafsanjani said this week, according to the Mehr news agency. "If circumstances are such that there will be conflicts and disputes between me and the leader, we will all lose."

He was president from 1989 to 1997. Khatami, a reformist who succeeded Rafsanjani until 2005, retains much popularity, especially among secular middle classes, youth and women voters.

"Khatami is definitely the most loved," said a social media consultant in Tehran working for a reformist campaign. "Everyone is waiting for his response. The Qalibaf and Velayati staff are calling his office constantly to see if he will run."

Both Rafsanjani and Khatami have been sidelined since 2009 and their association with reformist leaders Mirhossein Mousavi and Mehdi Karoubi - losing candidates to Ahmadinejad in 2009 and under house arrest for more than two years - has attracted sharp criticism from hardliners.

Mainstream contenders are not the only ones facing pressure from the authorities. Hooshang Amirahmadi, an academic living in the United States, intends to take part in the election to call for economic reform and ending confrontation with Washington but said that officials have tried to discourage him from standing.

"I was in Tehran two weeks ago and they told me to stay away," he told Reuters from New Jersey, where he is a professor of planning and public policy at Rutgers University.

"They said that my safety is in danger. They are really nervous and that surprises me."