Reformists led by ex-president Mohammad Khatami have rallied behind the sole moderate candidate standing in Friday's presidential vote, as they seek to loosen the conservative grip on power in Iran.

Khatami urged voters to support Hassan Rowhani hours after the only reformist candidate Mohammad Reza Aref pulled out of the race.

"Due to the duty I have to the country and the fate of the people of Iran I will vote for my esteemed brother Hassan Rowhani," Khatami said on his website.

"I ask all, especially reformists and those who seek the dignity and elevation of the nation," to vote for Rowhani "in order for their demands to be met."

Rowhani also won a second major endorsement from pragmatic former president Akbar Hashemi Rafsanjani, who said the former nuclear negotiator who champions better international ties is "more suitable" for the post.

"I will vote for Dr Rowhani, who entered the race after consulting me," Rafsanjani said in remarks reported by the ILNA news agency on Tuesday.

The reformist support is seen as improving the chances of Rowhani, 64, of mounting a credible bid against the other major contenders, mostly conservatives seen as close to supreme leader Ayatollah Ali Khamenei.

The election is for a successor to President Mahmoud Ahmadinejad, whose eight years in office have been marked by stiff Western sanctions over Tehran's controversial nuclear drive and the economic turmoil they have caused.

On Monday, ex-parliamentary speaker Gholam Ali Haddad Adel, also seen as close to Khamenei, pulled out of the race, saying he was "promoting the chances of a conservative election."

Frontrunners in the election are hardline conservative Saeed Jalili, former foreign minister Ali Akbar Velayati and Tehran's mayor Mohammad Baqer Qalibaf.

Fellow hopefuls Mohsen Rezai and Mohammad Gharazi are trailing in the polls.

Aref on Tuesday announced he was quitting after Khatami had sent him a letter advising that it "would not be wise" for him to continue.

Khatami's intervention came amid growing calls in recent days from within the reformist camp for Aref to stand aside in favour of Rowhani.

Khatami's advisory council had already said it was backing Rowhani as the "reformist camp candidate."

Rowhani had been top nuclear negotiator under Khatami and served as a senior security official under Rafsanjani.

"I will follow the paths of Khatami and Rafsanjani," the media quoted Rowhani as saying on Monday at a rally in the western province of Kurdistan.

"I do not approve of the current foreign policy. We should try to have good (international) interactions to gradually reduce the sanctions and finally remove them," he added.

Rowhani has advocated flexibility in negotiations over Iran's controversial nuclear drive, which world powers believe is masking an atomic weapons programme despite repeated denials by Tehran.

When Rowhani was Iran's top nuclear negotiator, Tehran agreed to suspend uranium enrichment and to allow snap inspections by the UN atomic watchdog in accordance with the nuclear non-proliferation treaty.

Supporters of Jalili, head of the Supreme National Security Council and tasked with negotiating with world powers, have subsequently harshly criticised Rowhani's approach during his tenure, saying he made too many concessions in return for too little.

Rowhani quit his post in 2005 over disagreement with Ahmadinejad, as Iran resumed the sensitive uranium enrichment work that it had suspended.

Pro-reformist and moderate newspapers hailed Aref's withdrawal.

The reformist Arman ran the headline "Aref greets Rowhani... the great consensus."

The Etemad newspaper ran its main story as: "Reformist votes Rowhani," adding that the decision follows weeks of deliberations.

Aref's move has prompted the conservative camp to call for union among the remaining conservative candidates.

"In case of division of the vote among the conservatives, then the conservative candidate will be elected with just a few more votes than his reformist rival," wrote the publisher of the hardline Kayhan newspaper, Hossein Shariatmadari.

The Guardians Council, an unelected vetting body, had approved only eight men out of 686 hopefuls to stand in the election, and the list is dominated by conservatives.

Rafsanjani, a prominent figure in the Iranian revolution who served as president from 1989 to 1997, was surprisingly barred from running, as was Esfandiar Rahim Mashaie, a divisive figure close to the incumbent.

Popular reformist candidates in the 2009 election, Mir Hossein Mousavi and Mehdi Karroubi, have remained under house arrest for more than two years.

Both had claimed Ahmadinejad's re-election was fraudulent, sparking months of protests by their supporters that were eventually crushed by security forces.