Rights groups have expressed alarm that Iranian authorities were intensifying a deadly crackdown under cover of an internet blackout, after another night of mass protests in the biggest demonstrations to face the Islamic republic in years.

The two weeks of demonstrations have posed one of the biggest challenges to the theocratic authorities who have ruled Iran since the 1979 Islamic revolution, although Supreme Leader Ayatollah Ali Khamenei has expressed defiance and blamed the United States.

Following the movement’s largest protests yet on Thursday (local time), new demonstrations took place late on Friday, according to images verified by AFP and other videos published on social media.

This was despite an internet shutdown imposed by the authorities, with monitor Netblocks saying Saturday evening that “Iran has now been offline for 48 hours”.

Amnesty International said it was analysing “distressing reports that security forces have intensified their unlawful use of lethal force against protesters” since Thursday in an escalation “that has led to further deaths and injuries”.

Norway-based Iran Human Rights group has said at least 51 people have been killed in the crackdown so far, warning the actual toll could be higher.

The nationwide protests started in Tehran's Grand Bazaar against the failing economic policies in late December, which spread to universities and other cities. Photo / Mahsa, Middle East Images via AFP

It posted images it said were of bodies of people shot dead in the protests on the floor of Alghadir hospital in eastern Tehran.

“These images provide further evidence of the excessive and lethal use of force against protesters,” IHR said.

‘Seize city centres’

In Tehran’s Saadatabad district, people banged pots and chanted anti-government slogans including “death to Khamenei” as cars honked in support, a video verified by AFP showed.

Other images disseminated on social media and by Persian-language television channels outside Iran showed similarly large protests elsewhere in the capital, as well as in the eastern city of Mashhad, Tabriz in the north and the holy city of Qom.

In the western city of Hamedan, a man was shown waving a shah-era Iranian flag featuring the lion and the sun amid fires and people dancing.

The same flag briefly replaced the current Iranian flag over the country’s embassy in London, when protesters managed to reach the building’s balcony, witnesses told AFP.

Reza Pahlavi, the US-based son of Iran’s ousted shah, hailed the “magnificent” turnout on Friday and urged Iranians to stage more targeted protests on Saturday and Sunday.

“Our goal is no longer just to take to the streets. The goal is to prepare to seize and hold city centres,” Pahlavi said in a video message on social media.

‘Big trouble’

Pahlavi, whose father Mohammad Reza Pahlavi was ousted by the 1979 revolution and died in 1980, added he was also “preparing to return to my homeland” at a time that he believed was “very near”.

Authorities say several members of the security forces have been killed, and Khamenei in a defiant speech on Friday lashed out at “vandals” and accused the United States of fuelling the protests.

On Thursday and Friday, an AFP journalist in Tehran saw streets deserted and plunged into darkness before any protests.

On Valiasr Ave, one of Tehran’s main streets, businesses shuttered unusually early.

“The area is not safe,” said a cafe manager as he prepared to close around 4pm.

An AFP reporter saw shop windows broken, as well as security forces deploying.

State TV on Saturday broadcast images of funerals for several members of the security forces killed in the protests, including a large gathering in the southern city of Shiraz.

It also aired images of buildings, including a mosque, on fire.

Iran’s army said in a statement that it would “vigorously protect and safeguard national interests” against an “enemy seeking to disrupt order and peace”.

National security council chief Ali Larijani said in comments broadcast late Friday that “we are in the middle of a war”, with “these incidents being directed from outside”.

The Norway-based Hengaw rights group said it had confirmed five Kurdish men had been shot dead by security forces in the western city of Kermanshah on Thursday and another man, a former bodybuilding champion, killed in the northern city of Rasht on Friday.

Global leaders have urged restraint from Iranian authorities, with European Union chief Ursula von der Leyen saying Europe backed Iranians’ mass protests and condemned the “violent repression” against the demonstrators.

On Saturday, the start of the working week in Iran, one man in Tehran said he was unable to check his work email.

“This is the price to pay before the victory of the people,” he said.

US President Donald Trump again refused on Friday to rule out new military action against Iran after Washington backed and joined Israel’s 12-day war against the Islamic republic in June.

“Iran’s in big trouble. It looks to me that the people are taking over certain cities that nobody thought were really possible just a few weeks ago,” Trump said.

- Agence France-Presse