This nerve centre in the campaign to unseat hardline Iranian President Mahmoud Ahmadinejad is not run by any of the three candidates who are challenging him in the election on Friday. Instead, it is part of a bitter behind-the-scenes rivalry that has helped define the campaign, pitting Ahmadinejad against the man he beat in the last election, Ali Akbar Hashemi Rafsanjani, a two-term former President and one of Iran’s richest and most powerful men.

On Tuesday, Rafsanjani, who is backing the campaign against the President and whose son runs the war room, released an open letter in which he complained about what he called the President’s “insults, lies and false allegations” and asked Iran’s supreme leader Ayatollah Ali Khamenei to intervene. During a campaign debate with the strongest challenger a week ago, Ahmadinejad accused Rafsanjani of stealing billions of dollars of state money and called him “the main puppet master” behind the campaign against him.

The President clearly hopes to associate his challengers with Rafsanjani, who is one of the Islamic republic’s founding figures but is widely viewed as corrupt.

Rafsanjani is striking back, accusing Ahmadinejad of undermining the state itself. His letter casts Ahmadinejad’s election broadsides — aimed at several figures who were close to Ayatollah Ruhollah Khomeini, the father of the 1979 revolution — as an attack on the country’s senior political class and therefore on the legitimacy of the entire system.

“If the system cannot or does not want to confront such ugly and sin-infected phenomena as insults, lies, and false allegations made in that debate, how can we consider ourselves followers of the sacred Islamic system?” Rafsanjani wrote.

No sooner had Rafsanjani’s letter been released than Ahmadinejad’s campaign began copying and distributing it, clearly betting that it would play in the President’s favour to keep Rafsanjani in the spotlight, said Mohammad Ali Abtahi, the President of the Institute for Interreligious Dialogue, a research organisation here.

Thousands of Ahmadinejad’s supporters marched through central Tehran on Wednesday morning. Hours later, a larger throng of supporters of the leading challenger, Mir Hussein Moussavi, rallied in the capital, with tens of thousands of people bearing the signature green headbands, ribbons and caps of his campaign.

Although the rallies have been mostly peaceful, the enormous street presence by Moussavi’s supporters has clearly rattled some in the Government. On Wednesday, a senior official of Iran’s Revolutionary Guards accused the Moussavi campaign of trying to start a “velvet revolution”, according to comments published on the Guards’ website.

In his letter, Rafsanjani noted that Ayatollah Khamenei had “deemed it best to remain silent” instead of censuring the President for his vitriolic attacks during the debate. Rafsanjani said he wrote the letter only after the rejection of his demands for an apology and for an opportunity to rebut the charges on state television.

Khamenei is unlikely to respond because “he is not pleased with correspondence like this from anyone”, Abtahi said.

Rafsanjani’s letter is especially significant because he leads the Assembly of Experts, an 86-member body of senior clerics that has the power to remove the supreme leader, Abtahi said. It included a veiled threat: Rafsanjani implicitly compared Ahmadinejad to a former President whom Rafsanjani helped depose in 1981.

In this year’s campaign, Moussavi, a former Prime Minister with a reputation for honesty and competence, has emerged as Ahmadinejad’s strongest challenger.

Moussavi and others have derided the President’s efforts to associate them with Rafsanjani. But Rafsanjani’s family is playing a powerful role in supporting Moussavi’s campaign — whether by invitation or not.

Rafsanjani’s 39-year-old son, Mehdi Hashemi Rafsanjani, directs the sophisticated electoral effort based at Islamic Azad University, which was founded by his father. It is here that the young women, paid $20 to $25 a day, can be seen at all hours assembling data to help election monitors observe the vote on Friday.