In the past few months, Iranian rights activists have urged women to publicly remove their veils, a gesture that would risk their arrest for defying the Islamic dress code as the country's hardline rulers crack down harder on "immoral behaviour".

Videos posted on social media have shown cases of what appeared to be heavy-handed action by morality police units against women who had removed their hijab.

Authorities launched probes into the death of Mahsa Amini following a demand by President Ebrahim Raisi, state media reported. Police said the 22-year-old was taken ill as she waited together with other detained women at a morality police station.

"Since her transfer to the vehicle and also at the location (station), there was no physical encounter with her," a police statement said, rejecting allegations on social media that Amini was likely beaten.

Closed-circuit television footage carried by state TV appeared to show a woman identified as Amini falling over after getting up from her seat to speak to an official at a police station. Reuters could not authenticate the video.

Police earlier said Amini had suffered a heart attack after being taken to the station to be "educated". Her relatives have denied she suffered any heart condition.

Several prominent sports and arts figures posted critical social media comments about Amini's death, and outspoken reformist politician Mahmoud Sadeghi called on Supreme leader Ayatollah Ali Khamenei on Twitter to speak out as he had denounced the killing of George Floyd by U.S. police in 2020.

Postings on social media included videos showing protesters chanting "Death to the dictator (Khamenei)" as drivers sounded their car horns to back protests in a Tehran square near Amini's hospital amid a heavy police presence.

As during past protests, authorities appeared to have restricted internet access in the capital Tehran to make it difficult for protesters to post videos on social media.

Internet blockage observatory NetBlocks reported on Twitter that there was "a significant internet outage" in Tehran, linking the incident to the protests.

U.S. special envoy for Iran, Robert Malley, said on Twitter: "Mahsa Amini’s death after injuries sustained in custody for an 'improper' hijab is appalling ... Those responsible for her death should be held accountable."

Rights group Amnesty International said on Twitter: "... allegations of torture and other ill-treatment in custody, must be criminally investigated ... All agents and officials responsible must face justice."

Under Iran's sharia (Islamic) law, imposed after the 1979 revolution, women are obliged to cover their hair and wear long, loose-fitting clothes to disguise their figures. Violators face public rebuke, fines or arrest.

Decades after the revolution, clerical rulers still struggle to enforce the law, with many women of all ages and backgrounds wearing tight-fitting, thigh-length coats and brightly coloured scarves pushed back to expose plenty of hair.