The leaders of France, the United Kingdom and Germany have condemned what they described as the "killing of protesters" in Iran, urging the authorities to "exercise restraint".
"We are deeply concerned about reports of violence by Iranian security forces, and strongly condemn the killing of protestors," French President Emmanuel Macron, UK Prime Minister Keir Starmer and German Chancellor Friedrich Merz said in a joint statement.
"We urge the Iranian authorities to exercise restraint," they added.
However, US President Donald Trump tonight again warned that he could order military strikes against Iran, saying the country was in "big trouble".
"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," Mr Trump said.
Asked on his message to Iran's leaders, he replied: "You better not start shooting because we'll start shooting too."
Mr Trump added: "If they start killing people like they have in the past, we will get involved.
"That doesn't mean boots on the ground, but it means hitting them very, very hard where it hurts."
Iran has been largely cut off from the outside world after authorities blacked out the internet to curb growing unrest, as video showed buildings ablaze in anti-government protests raging in several cities across the country.
Rights groups have already documented dozens of deaths of protesters in nearly two weeks and, with Iranian state TV showing clashes and fires, the semi-official Tasnim news agency reported that several police officers had been killed overnight.
In a televised address, Supreme Leader Ayatollah Ali Khamenei vowed not to back down, accusing demonstrators of acting on behalf of émigré opposition groups and the United States, and a public prosecutor threatened death sentences.
Chanting slogans including "death to the dictator" and setting fire to official buildings, crowds of people opposed to the leadership marched through major cities last night.
Internet monitor Netblocks said authorities had imposed a total connectivity blackout and added early Friday that the country has "now been offline for 12 hours... in an attempt to suppress sweeping protests".
Video has shown buildings and vehicles ablaze in anti-government protests raging through the streets of several cities.
The demonstrations represent one of the biggest challenges yet to the Islamic republic in its over four-and-a-half decades of existence, with protesters openly calling for an end to its theocratic rule.
But Mr Khamenei struck a defiant tone in his first comments on the escalating protests since 3 January, calling the demonstrators "vandals" and "saboteurs", in a speech broadcast on state TV.
Watch: Iranians rip national flag in half amid unrest
Mr Khamenei said US President Donald Trump's hands "are stained with the blood of more than a thousand Iranians", in apparent reference to Israel's June war against the Islamic republic which the US supported and joined with strikes of its own.
He predicted the "arrogant" US leader would be "overthrown" like the imperial dynasty that ruled Iran up to the 1979 revolution.
He said: "Last night in Tehran, a bunch of vandals came and destroyed a building that belongs to them to please the US president," he said in an address to supporters, as men and women in the audience chanted the mantra of "death to America".
"Everyone knows the Islamic republic came to power with the blood of hundreds of thousands of honourable people, it will not back down in the face of saboteurs."
Mr Trump said last night that "enthusiasm to overturn that regime is incredible" and warned that if the Iranian authorities responded by killing protesters, "we're going to hit them very hard. We're ready to do it."
In the Fox News interview, Mr Trump went as far as to suggest 86-year-old Mr Khamenei may be looking to leave Iran.
"He's looking to go someplace," he said.
Watch: Large crowds take to the streets in Ilam province
EU's Kallas says violence against Iran protests 'disproportionate'
European Union foreign policy chief Kaja Kallas said the response of the Iranian security forces to protests is "disproportionate" and added that any violence against peaceful demonstrators is unacceptable.
"Shutting down the internet while violently suppressing protests exposes a regime afraid of its own people," Ms Kallas said on social media platform X.
Iran's fragmented external opposition factions have called for more protests with Reza Pahlavi, exiled son of the late ruling shah, telling Iranians in a social media post: "The eyes of the world are upon you. Take to the streets."
Mr Trump, who bombed Iran last summer and who last week warned Tehran it could come to the protesters' aid, said he would not meet Mr Pahlavi,and was "not sure that it would be appropriate" to back him.
The images published by state television overnight showed what it said were burning buses, cars and motorbikes as well as fires at metro stations and banks.
It accused the People's Mujahedin Organisation, an opposition faction that splintered off after the 1979 Islamic Revolution and also known as the MKO, of being behind the unrest.
A state television journalist standing in front of the fires on Shariati Street in the Caspian Sea port of Rasht said "this looks like a war zone - all the shops have been destroyed".
Iran has quelled far bigger bouts of unrest before, but it now faces a graver economic situation and intensifying international pressure with global sanctions over its nuclear programme reimposed since September.
A French diplomatic source said Iran should show maximum restraint towards protesters.
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The protests have not yet approached the scale of nationwide demonstrations in late 2022 over women's rights following the death in custody of a Kurdish woman, Mahsa Amini, but they pose the biggest internal challenge to the authorities since then.
The authorities have attempted a dual approach - describing protests over the economy as legitimate while condemning what they call violent rioters and cracking down with security forces.
