Four children of an influential former Iranian president are suing a radical politician for describing his family as a corrupt "octopus".

The dispute has rekindled bitterness between backers of the current president, Mahmoud Ahmadinejad, and moderates headed by the former leader, Akbar Hashemi Rafsanjani, six months before the next presidential election.

One of the ex-president's sons, Mehdi Rafsanjani, was arrested in late September, a day after he returned to Iran from Britain, on charges of fomenting unrest in the aftermath of Iran's disputed 2009 presidential election. He was released from Evin prison on bail this month, and has not been put on trial.

In a speech broadcast on state radio, the politician and cleric Hamid Rasai, an ally of Ahmadinejad, called Mehdi a "corrupt monster who has always enjoyed ironclad immunity". Rasai urged the judiciary to deal with him harshly, calling Rafsanjani and his relatives an "octopus family" that pressured judiciary and security bodies to free Mehdi.

"Four children of Ayatollah Rafsanjani have registered their lawsuit against Rasai with the special clergy court. I think Rasai will be summoned to the court within the next 10 days," said the lawyer Mahmoud Alizadeh Tabatabaei, according to the semi-official ISNA news agency.

Since the 2009 election, in which Rafsanjani supported the reformist challenger Mir Hossein Mousavi, the family has come under pressure from hardliners. Rafsanjani's youngest daughter, Faezeh, is serving a six-month sentence on charges of distributing propaganda against Iran's ruling system.

Many analysts believe Ahmadinejad won the 2009 vote partly because he portrayed himself as a champion of the poor and called Rafsanjani a symbol of aristocracy. He also called Mousavi a protege of Rafsanjani.

In recent months there have been indications that the 78-year-old Rafsanjani, who favours a more moderate approach to the west, might try to make a political comeback.

Iran's judiciary rejected Rasai's claims, calling a large part of his remarks "sheer lies". In a statement, it said Rasai's remarks were "criminal" and must be dealt with by the court.

Several politicians responded on Wednesday, claiming Rasai had parliamentary immunity. In the past the judiciary has imprisoned politicians for making accusations against individuals who had not been convicted in court, saying parliamentary immunity did not allow them to terrorise an innocent citizen.

Rafsanjani's family provided a written response to Rasai, which was posted on Rafsanjani's website on Wednesday. "It is expected that the respected special clergy court, should it find these remarks a kind of encroachment on the position of others, will take legal action … because no one has the right to attribute crimes to a defendant who has not been sentenced in a competent court," the family said in its letter.