Norway-based Iran Human Rights (IHR) said the figure, based on state media reports and its own research, represented a minimum given the current internet restrictions in the Islamic republic, and that the actual number of arrests was "likely much higher".

It said at least 3,646 people had been arrested since the war broke out on February 28, with at least 767 of the cases reported after the start of a ceasefire on April 8.

"The charges levelled against the detainees predominantly include espionage, communicating with foreign intelligence services, transmitting images or coordinates of sensitive locations to foreign-based media, and attempting to establish operational cells or conduct armed activities," it said.

People have also been arrested for using and distributing Starlink satellite-based internet terminals to circumvent internet blackouts, and for alleged cooperation with pro-monarchist groups.

IHR said more than 100 civil society activists were among those arrested, including prize-winning rights lawyer Nasrin Sotoudeh, who was detained on April 2.

Sotoudeh's daughter Mehraveh Khandan wrote on Instagram on Saturday that her mother had telephoned for the first time since her arrest, saying she was being held by the intelligence ministry but was not allowed to disclose where.

Her fellow rights activist and 2023 Nobel Peace Prize winner Narges Mohammadi remains in prison after her arrest in December, which took place before the war and mass protests in January.

Mohammadi on Tuesday turned 54 behind the bars of her prison in the northern city of Zanjan, her foundation said, after warning last week her health was "critical" following a heart attack in March.