The daughter of Ayatollah Ruhollah Khomeini, the founder of Iran's Islamic republic, has protested against the disqualification of Akbar Hashemi Rafsanjani from next month's presidential election.

Zahra Mostafavi has written to the country's supreme leader, Ayatollah Ali Khamenei, calling on him to reinstate Rafsanjani in order to prevent the forming of a dictatorship.

Neither Rafsanjani, a confidant of Khomeini and the opposition's favoured candidate, nor President Mahmoud Ahmadinejad's close ally Esfandiar Rahim Mashaei were among the eight candidates approved by the guardian council to enter the ballot out of at least 680 people who registered this month.

Ahmadinejad expressed dissatisfaction with the decision against Mashaei, describing it as an act of oppression. He promised to raise the issue with Khamenei.

"In my opinion there will be no problem with the supreme leader and I will take up this issue until the last moment with him," Ahmadinejad said on Wednesday. "I am hopeful the problem will be solved."

The guardian council, a powerful body of six jurists and six clergymen, vets all candidates to make sure they have sufficient loyalty to the Islamic republic and its principles, but critics say the group's function is undemocratic.

It does not publicly give reasons for its decisions but Rafsanjani, 78, is said to have been barred because he was considered too old and Mashaei for being a nationalist figure. Rafsanjani's moderate support for the opposition Green movement is also believed to have counted against him.

Unlike Mashaei's rejection, which was widely expected, Rafsanjani's exclusion has come as a surprise to many, especially supporters of the Islamic republic who regard him as one of Iran's great political survivors, formerly serving as president for two consecutive terms.

Rafsanjani's supporters see many ironies in his disqualification, not least that he is currently head of the expediency council, which mediates between the guardian council and parliament. Two of the guardian council's members are older than Rafsanjani, and Khomeini was not much younger when he took power in 1979.

In Mostafavi's letter – published on Jamaran.ir, a website close to Khomeini's family – she wrote: "Unfortunately I see that the guardian council has blocked him [Rafsanjani] for presidency … This act has no meaning other than creating a separation between two companions of the imam [Khomeini] and a disregarding of the enthusiasm and interest of the people towards the system and the elections,"

She wrote that Khomeini had thought of Rafsanjani as a potential candidate to succeed him as supreme leader, and cited a saying by her father with the message that the supreme leader should prevent any occurrence of dictatorship.

"The gradual separation between the two of you [Khamenei and Rafsanjani] will be the biggest blow to the revolution and the system," she wrote. "The imam always said: 'These two are good when they are together.'"

Rafsanjani has yet to respond to his disqualification but Eshaq Jahangiri, the head of his official campaign, told the Isna news agency that he would not object.

Not all of Rafsanjani's supporters were upset with his disqualification. One said it showed "the empress has no clothes", referring to how Khamenei was tightening his grip on Iranian politics.

Abolhassan Banisadr, Iran's first post-revolution president, who now lives in exile, told the BBC's Persian service that Rafsanjani's last political gamble had paid off by exposing the establishment's increasing dogmatism.

Karim Sadjadpour, of the Carnegie Endowment for International Peace, tweeted: "Increasingly looking like Iran's presidential election will be one man, one vote. That one man's name is Ayatollah Khamenei."

Wednesday's newspapers in Tehran largely avoided the disqualifications and instead focused on the eight men allowed to stand.

Tehran residents on social networking websites reported a heavy presence of security forces on the streets on Tuesday night as the final candidate list was announced on state-run TV, and complained of an increase in online censorship and slower internet connections.

The final list of candidates includes few reformist figures but they seem to have little chance of victory. Tehran's mayor, Mohammad Bagher Ghalibaf, and Iran's chief nuclear negotiator, Saeed Jalili, who is seen as Khamenei's favourite candidate, are on the list.

Ali Motahari, a Tehran MP whose father was a prominent revolutionary figure, objected to Jalili's candidacy, saying he had little experience in executive positions.

Many analysts believe that the disqualifications have smoothed the path for Jalili to potentially succeed Ahmadinejad. Mehdi Khalaji, a senior fellow at the Washington Institute for Near East Policy, said on Tuesday that Khamenei was sending a message that Tehran would not compromise over its nuclear programme.