File Photo: Iranian reformist politician Mostafa Tajzadeh speaks to the media after registering his candidacy at the Interior Ministry in the capital Tehran, for the Islamic Republic s upcoming presidential elections. AFP
"In the name of the Reform Front of Iran, I urge you to release (Tajzadeh) as soon as possible," the head of the coalition, Behzad Nabavi, wrote in an open letter to the judiciary.
He called for Tajzadeh's case to be "examined in an open court session" by an "impartial" prosecutor, according to the letter published in local media.
Formed in March 2021 by close associates of reformist former president Mohammad Khatami, the Front brings together parties and movements from Iran's reformist camp.
Tajzadeh served as deputy interior minister under Khatami, who held office from 1997 to 2005.
The 65-year-old's trial began last week at a branch of Iran's Revolutionary Court.
According to the judicial authority's news agency Mizan Online, Tajzadeh faced "three counts, including conspiracy against national security".
He also faces accusations of "publishing lies to disturb public opinion", the Mehr news agency has reported.
Tajzadeh had previously served seven years in prison, having been arrested in 2009 during protests disputing the re-election of then president Mahmoud Ahmadinejad.
He declined to speak in court during last week's hearing after a request for him to talk one-on-one with his lawyer was rejected, the defence said.
Tajzadeh "did nothing but express his opinions", the Reform Front said, adding that he has been held in "solitary confinement" since his arrest.
He had made an unsuccessful attempt to run in Iran's presidential elections in 2021, but his candidacy was rejected.
The Iranian authorities have made several high-profile arrests in recent months, including of acclaimed filmmakers Jafar Panahi and Mohammad Rasoulof.