The official death toll from Iran's wave of popular unrest shot up Thursday to at least 17 as popular anger has flared over the death in custody of the young woman Mahsa Amini.

However, the Oslo-based non-government group Iran Human Rights said at least 31 civilians had been killed in a crackdown by the Iranian security forces in six nights of violence.

Iranians have taken to the streets "to achieve their fundamental rights and human dignity... and the government is responding to their peaceful protest with bullets," charged its director Mahmood Amiry-Moghaddam.

The Kurdish woman Amini, 22, died last week after she had been arrested by the Iran’s feared morality police for allegedly wearing a hijab headscarf in an "improper" way, sparking widespread outrage.

"Death to the dictator" and "Woman, life, freedom", protesters could be heard shouting in video footage shared online, during the biggest wave of protests to rock the country in almost three years.

Among those killed in clashes have been police and militia officers, state TV reported, while overseas-based human rights groups reported many more deaths, which could not be independently verified.

Security forces have fired at crowds with birdshot and metal pellets, and also deployed tear gas and water cannons, according to Amnesty International and other human rights groups.

There were fears violence could escalate further after Iranian authorities restricted internet access and blocked messaging apps including WhatsApp and Instagram, as they have done before past crackdowns.

Some women have burnt their scarves and symbolically cut their hair in protest at the strict dress code, in defiant actions echoed in solidarity protests abroad from New York to Istanbul.

Activists have said that Amini, whose Kurdish first name is Jhina, after her detention in Tehran suffered a fatal blow to the head -- a claim denied by officials, who have announced an investigation.