Iran is using drones and intrusive digital technology to crush dissent, especially among women who refuse to obey the Islamic republic's strict dress code, the United Nations has said.

Investigators say Iranian security officials are using a strategy of "state-sponsored vigilantism" to encourage people to use specialist phone apps to report women for alleged dress code violations in private vehicles such as taxis and ambulances.

Their new report also highlights the increasing use of drones and security cameras to monitor hijab compliance in Tehran and in southern Iran.

For women who defy the laws, or protest against them, the consequences are severe – arrest, beating, and even rape in custody.

The findings of the Independent International Fact-Finding Mission on the Islamic Republic of Iran come after it determined last year that the country's theocracy was responsible for the "physical violence" that led to the death in custody of Mahsa Amini in 2022.

Witnesses said the 22-year-old Kurd was badly beaten by the morality police during her arrest, but authorities denied she was mistreated and blamed "sudden heart failure" for her death. Her killing sparked a massive wave of protests that continues today, despite threats of violent arrest and imprisonment.

"Two-and-a-half years after the protests began in September 2022, women and girls in Iran continue to face systematic discrimination, in law and in practice, that permeates all aspects of their lives, particularly with respect to the enforcement of the mandatory hijab," the report said.

"The state is increasingly reliant on state-sponsored vigilantism in an apparent effort to enlist businesses and private individuals in hijab compliance, portraying it as a civic responsibility."

At Tehran's Amirkabir University, authorities installed facial recognition software at its entrance gate to also find women not wearing the hijab, the report said.

Surveillance cameras on Iran's major roads are also being used to search for uncovered women.

Investigators also said they obtained the "Nazer" mobile phone app offered by Iranian police, which allows "vetted" members of the public and the police to report on uncovered women in vehicles, including ambulances, buses, metro cars and taxis.

"Users may add the location, date, time and the licence plate number of the vehicle in which the alleged mandatory hijab infraction occurred, which then 'flags' the vehicle online, alerting the police," the report said.

According to the report, a text message is then sent to the registered owner of the vehicle, warning them they had been found in violation of the mandatory hijab laws. Vehicles could be impounded for ignoring the warnings, it added.

The UN investigators interviewed almost 300 victims and witnesses – they also looked in-depth at Iran's judicial system, which they said lacks any real independence. Victims of torture and other violations were also persecuted while their families were "systematically intimidated", according to their report.

They also found evidence of the extrajudicial executions of three child and three adult protesters, later dismissed by the state as suicides.

The report also established additional cases of sexual violence in custody, citing the case of one arrested woman who was beaten severely, subjected to two mock executions, raped and then gang-raped.

The report will be presented to the Human Rights Council on 18 March.