"Women, Life, Freedom!" protesters chanted on Thursday night in Sanandaj, the capital of Amini's home province of Kurdistan in northwest Iran, before the crackle of gunfire was heard, in a video posted on Twitter by the Iran Wire news website.
Crowds were seen "chanting slogans and confronting security forces" in cities across the country, including Tehran, Isfahan and Yazd, said the site run by Iranian journalists based abroad.
In the shrine city of Mashhad, protesters clashed with security forces, hurling stones at them and shouting: "We will kill whoever killed my sister!"
"Thousands of people have been arrested and dozens... have been killed. Journalists are in jail. Students, professors, truck drivers and many others have gone on strike," Iran Wire reported.
Women have defiantly burned their headscarves and cut their hair in the protests over Amini's death, announced September 16, that are the biggest seen in Iran since protests in November 2019 over fuel price rises.
Amnesty International said Iran was intentionally using lethal force against the protesters, adding that without international action "beyond statements of condemnation" more people risk being killed.
"The Iranian authorities have mobilised their well-honed machinery of repression to ruthlessly crack down on nationwide protests in an attempt to thwart any challenge to their power," it said.
Order to confront protests 'mercilessly'
Amnesty said it had obtained a leaked official document issued to the commanders of armed forces in all provinces on September 21 instructing them to "severely confront" protesters.
Another leaked document showed that on September 23, the commander of the armed forces in Mazandaran province, where some of the deadliest clashes have taken place, ordered security forces to "confront mercilessly, going as far as causing deaths, any unrest by rioters and anti-revolutionaries".
The NGO said it had confirmed 52 deaths in the protests but the toll was likely higher. Another rights group, Oslo-based Iran Human Rights, said 83 people were confirmed to have been killed, while Iran's Fars news agency has put the toll at "around 60" dead.
Amnesty's warning comes as Iran presses ahead with an intensifying crackdown that has seen the arrest of many journalists, activists and other prominent figures.
Former Iranian international footballer Hossein Manahi was arrested Friday after supporting the protests on his social media accounts, the state-run IRNA news agency said.
Security forces also arrested singer Shervin Hajipour, whose song "Baraye" ("For") made up of tweets about the protests went viral on Instagram, the rights group Article 19 and Persian-language media based outside Iran said.
His song, which racked up millions of views on Instagram and prompted many to comment that it moved them to tears, has now been removed from his account.
The Washington-based Committee to Protect Journalists says it has documented the arrest of at least 29 journalists.
These include two female reporters, Nilufar Hamedi and Elahe Mohammadi, who helped expose the case of Amini to the world by reporting respectively from her hospital and her funeral.
The crackdown has drawn widespread international condemnation.
Solidarity protests with Iranian women have been held worldwide, and rallies are planned in 77 cities on Saturday.
Iran has blamed outside forces for the protests, and on Wednesday it launched cross-border missile and drone strikes that killed 14 people in Iraq's Kurdistan region, accusing exiled armed groups based there of fuelling the unrest.
Iran's Revolutionary Guards pledged to carry out more attacks until the groups are disarmed.
The United States on Thursday said one of its citizens had been killed in the Iranian strikes, separately announcing the fresh enforcement of sanctions on the Islamic republic's oil sales.
Iran's economy is already hit by punishing sanctions over its contested nuclear programme.
On Thursday, German Foreign Minister Annalena Baerbock said she was "doing everything" she could to push for European Union sanctions against those "beating women to death and shooting demonstrators in the name of religion".