"The culture and Islamic guidance ministry asked the judiciary and the security authorities about the arrest of Mr Panahi and they told us that it is a security case," Mohammad Hosseini told ISNA.
"They informed us that this director was making a film against the regime and it was about the events that followed election," he added, referring to the unrest which rocked Iran after the disputed re-election of President Mahmoud Ahmadinejad last year.
It is the first time that an Iranian official has given an explanation of the basis for Panahi's arrest.
But in an interview with AFP in mid-March, Panahi's wife, Tahereh Saeedi, denied that Panahi was making a film about post-election events, saying: "The film was being shot inside the house and had nothing to do with the regime."
She added that at that time prosecutors had yet to press any charges against her husband.
Soon after Panahi's arrest, Tehran prosecutor Abbas Jafari Dolatabadi said the filmmaker had not been detained for political reasons or because he was an artist but because he was "accused of some crimes and arrested with another person following an order by a judge."
According to opposition websites, Panahi was arrested along with 16 other people, including his wife and daughter and six human rights activists. Fourteen of those detained have been freed so far.
A vocal backer of the opposition movement, Panahi was arrested when security forces raided his Tehran home on March 1.
Fifty Iranian filmmakers and artists signed a letter in mid-March urging the authorities to release him.
Panahi, 49, is known for his gritty, socially critical movies such as "The Circle," which bagged the 2000 Venice Golden Lion award, "Crimson Gold" and "Offside," winner of the 2006 Silver Bear at the Berlin film festival.
In February, the authorities banned Panahi from leaving the country to attend the Berlin Film Festival.
