Iranian security forces opened fire on protesters who massed in their thousands on Wednesday in Mahsa Amini's hometown to mark 40 days since her death, a human rights group claimed.

"Security forces have shot tear gas and opened fire on people in Zindan square, Saqez city," Hengaw, a Norway-based group that monitors rights violations in Iran's Kurdish-populated regions, tweeted without specifying whether there were any dead or wounded.

Despite heightened security measures, columns of mourners had poured into Saqez in the western Kurdistan province to pay tribute to Amini at her grave at the end of the traditional mourning period.

Amini, a 22-year-old Iranian of Kurdish origin, died on Sept. 16, three days after her arrest in Tehran by the notorious morality police for allegedly breaching the Islamic dress code for women.

Anger flared at her funeral last month and quickly sparked the biggest wave of protests to rock the Islamic republic in almost three years. Young women have led the charge, burning their hijab headscarves and confronting security forces.

"Death to the dictator," mourners chanted at the Aichi cemetery outside Saqez, before many were seen heading to the governor's office in the city center.

Iran's Fars news agency said around 2,000 people gathered in Saqez and chanted "Woman, life, freedom".

But thousands more were seen making their way in cars, on motorbikes and on foot along a highway, through fields and even across a river, in videos widely shared online by activists and rights groups.

Noisily clapping, shouting and honking car horns, mourners packed the highway linking Saqez to the cemetery 8 kilometers (5 miles) away, in images that Hengaw told AFP it had verified.