Mahmoud Ahmadinejad's main reformist challenger said Sunday that the Iranian president has made false accusations against his supporters to try to sabotage his campaign with just days to go before Friday's presidential election.

Former Prime Minister Mir Hossein Mousavi wrote a letter to Iran's supreme leader, Ayatollah Ali Khamenei, accusing Ahmadinejad and his supporters of taking unethical steps against his campaign.

In an unusual twist for elections in Iran, the presidential campaign has descended into bitter personal attacks between candidates. Khamenei, who has final say on all state matters in Iran, has urged the contenders and their supporters to exercise restraint.

That call has gone unheeded, and a handful of political figures who are not even running in the campaign — as well as some of their relatives — are threatening to sue Ahmadinejad for publicly accusing them of corruption.

In the latest round, Mousavi's letter to the supreme leader said, "There is a possibility of fabrication of evidence against my supporters," according to several Iranian news reports published Sunday.

The reports gave no details of what Mousavi was alleging Ahmadinejad and his campaign have done specifically to target his supporters.

Officials with Mousavi's campaign confirmed Sunday that the letter was sent but refused to provide a copy to reporters or elaborate on its contents.

Ahmadinejad has previously accused Mousavi of having links with people involved in corruption.

Mousavi, who was prime minister during the country's years of war with Iraq in the 1980s, is among Iran's reformist camp, which seeks better ties with the West and an easing of social and political restrictions at home.

The hard-line Ahmadinejad's four years in office have been characterized by antagonism with the United States and its European allies over Iran's forward strides in its nuclear program, which they say is aimed at producing weapons. Iran denies the charge and says it only seeks peaceful nuclear energy.

Meanwhile, Mousavi's wife, Zahra Rahnavard, who has taken an unprecedented public role in her husband's campaign, threatened at a news conference on Sunday to sue Ahmadinejad for saying in a televised debate last week that she did not meet the full requirements to earn her Ph.D. in political science from Tehran's Azad University.

"I hired a lawyer and he will file a lawsuit if the president will not apologize for his immoral behavior," Rahnavard said.

She called Ahmadinejad a liar and gave him a deadline of noon Monday to make the apology before she files the lawsuit. Azad University said in a statement that she met all requirements to earn the degree in 1995.

"Today everyone is worried that they have a liar president," Rahnavard said.

During Wednesday's highly charged TV debate, Mousavi leveled accusations of his own, saying Ahmadinejad was driving Iran toward "dictatorship" and hurting its standing in the world by questioning the Holocaust.

Friday's election pits Ahmadinejad, with his deep support among Iran's poor, against Mousavi, who has criticized Ahmadinejad's stewardship of the country's economy.

In addition to Mousavi, Ahmadinejad's other reformist challenger is Mahdi Karroubi, a former parliamentary speaker. The sole conservative challenger is Mohsen Rezaei, a former Revolutionary Guards commander.