The run up to Friday's vote, which will herald an end to the difficult reformist presidency of President Mohammad Khatami, also remained darkened by bomb attacks -- with more blasts reported to have shaken the southeastern city of Zahedan.

As candidates accelerated their campaigns, Khatami advised voters to be sceptical of promises of more freedoms -- a clear snipe at the campaigns of religious hardliners busy reinventing themselves as slick moderates.

"Today all the candidates are talking about freedom, democracy, fighting censorship, the rights of the youth and women's rights," said Khatami, near the end of his second and final term in office.

"The important thing is to consider their records to see how committed they have been, and what practical plans they have to follow through with their promises."

Informal opinion polls in the Iranian press are signalling that none of the eight candidates will be able to secure more than 50 percent of the vote needed to win on June 17. In that case, the top two would face a run-off, which would be held on July 1.

Seen as trailing Rafsanjani in the number-two position is either former police chief Mohammad Baqer Qalibaf, a hardliner-turned smiling technocrat, and leftist-reformist Mostafa Moin.

But Rafsanjani, a former president who is presenting himself as a moderate with the clout to get things done, received a fresh boost on Tuesday.

The head of Iran's nuclear energy agency, Gholamreza Aghazadeh, told the Shargh and Jomhuri Islami newspapers that the cleric was "the only person capable of solving" Iran's nuclear stand-off with the international community.

Rafsanjani was "more competent that all of the other" seven candidates, Aghazadeh said, effectively confirming the prominent role the senior cleric has played in persuading the regime to enter into long-term negotiations with Britain, France and Germany.

Iran denies charges that it is seeking nuclear weapons, but has opted to suspend its nuclear fuel work and cooperate with United Nations inspectors.

"The nuclear programme, it's him," Aghazadeh said. "Rafsanjani offers the best chance to solve the domestic and international problems."

Throughout his campaign, Rafsanjani has been seizing Iran's political centre and driving the debate on key issues, including relations with the United States. He has said he is open to resuming ties, even though he says Washington has to take the first step by releasing billions of dollars of Iranian assets frozen since 1979.

Iran's Oil Minister Bijan Namdar Zanghaneh also threw his weight behind Rafsanjani by describing him as a "valued leader", and lashed out at hardliners "who used to say that ties and short sleeves are bad but now use them to get votes."

Meanwhile, the final days before the vote featured more bomb blasts -- albeit minor, amateurish attacks.

Several people were lightly wounded in three explosions in the southeastern city of Zahedan, the official news agency IRNA said. It said that low power blasts took place on Monday evening and early Tuesday morning.

Up to 10 people were killed in separate, larger attacks in the ethnic Arab-dominated city of Ahvaz and the capital Tehran on Sunday.

Judicial spokesman Jamal Karimi Rad told AFP that six suspects had been arrested on suspicion of involvement in the Ahvaz attacks and one over the Tehran blasts.

Officials have pointed the finger at the Iraq-based People's Mujahedeen, which is Iran's main armed opposition group, and Arab separatists linked to Baathist supporters of deposed Iraqi president Saddam Hussein.

But President Khatami played down earlier allegations of possible US or British involvement.

"We haven't discovered foreign footprints, but there have been provocations," he said. "What happened was a string of blind acts. At the moment good clues have been found, people have been arrested. We have to investigate."