The head of the department of Islamic guidance in the Iranian city of Mashhad has been arrested after people danced at an event in a local shopping centre.
A video showed a crowd of men and women enjoying a performance by a singer.
Mixing between the genders and dancing in public are severely restricted under Iran's Islamic laws.
A judicial official said the shopping centre event, which was authorised by the Islamic guidance department, was an "offence against public decency".
Mashhad is the location of an important Shia shrine and is a stronghold of particularly hardline clerics.
The video of the shopping centre event shows hundreds of men and women crowded onto several floors around an atrium, where a male pop singer is performing. As they watch, some clap and dance a little.
After the footage was shared online, hardliners in Mashhad expressed outrage and called for those who authorised the event to be held accountable.
On Thursday, the judiciary's news agency quoted deputy prosecutor Hassan Heydari as saying the head of the Islamic guidance department in Mashhad had been arrested for "undermining public decency and disrespecting the laws".
Mr Heydari told Fars news agency that the video had "created a wave of unhappiness among our citizens" and that prosecutors would "take action against all those involved".
A spokesman for the shopping centre, Mohsen Afshar, told Isna news agency that it had been holding similar events every month and that the singer had official permits allowing her to perform.
In March, the reformist mayor of Tehran, Mohammad Ali Najafi, submitted his resignation after hardliners criticised him for attending a school performance for International Women's Day in which young girls danced.
Mr Najafi's resignation was initially rejected by Tehran city council. But last week he resubmitted his resignation, citing ill health, and the council accepted it.
In a separate development on Thursday, the interior minister ordered an investigation after a video emerged allegedly showing a young woman being mistreated by a female morality police officer, who accused her of not covering her hair sufficiently at a Tehran park.
Disturbing video morality police beating a woman for “inadequate” hijab. Govt ministers say they will investigate but WE want compulsory hijab to be gone— masih alinejad (@AlinejadMasih) April 19, 2018
مسوولان: ویدیوی کتک زدن توسط #گشت_ارشاد را پیگیری میکنیم. ما: حجاب اجباری باید برود
Vice-President for Women and Family Affairs Masoumeh Ebtekar said the police's behaviour towards the woman was "unjustified". "Such harsh and irreligious behaviours are below the dignity of any human being," she tweeted.
Under Iranian law, women can be arrested for failing to cover their hair in public. Dozens of women have been detained in recent months for protesting against the law by removing their headscarves,