Iran has also sent a letter to the United Nations Security Council blaming the US for turning the demonstrations into what it called “violent subversive acts and widespread vandalism”.

US President Donald Trump, meanwhile, said Iran was “in big trouble”.

Now in their 13th day, the protests initially erupted over economic grievances but have since grown into the largest unrest seen in years, with demonstrators calling for an end to the Islamic Republic and some urging the restoration of the monarchy. At least 48 protesters and 14 members of the security forces have been killed, according to human rights groups.

An internet blackout is in place across much of the country. “We will be hitting them very hard where it hurts,” Trump said at the White House on Friday, adding that his administration was monitoring the situation in Iran closely and that any US involvement would not mean “boots on the ground”. “It looks to me like the people are taking over certain cities that nobody thought were really possible just a few weeks ago,” he said.

The remarks echoed comments made by the US president on Thursday, when he pledged to “hit them very hard” if the Iranian authorities “start killing people”.

Khamenei remained defiant in a televised address on Friday. “Let everyone know that the Islamic Republic came to power through the blood of several hundred thousand honourable people, and it will not back down in the face of those who deny this,” the 86-year-old said.

Later, in remarks delivered to a gathering of supporters and broadcast on state television, Khamenei doubled down, warning that Iran “will not shirk from dealing with destructive elements”.

Iran’s ambassador to the United Nations accused the United States of “interfering in Iran’s internal affairs through threats, incitement, and the deliberate encouragement of instability and violence” in the letter sent to the Security Council.

Since the protests began on 28 December, more than 2,277 people have been arrested, according to the US-based Human Rights Activists News Agency. The Norway-based Iran Human Rights group  said at least 51 protesters, including nine children, had been killed. BBC Persian has spoken to the families of 22 of them and confirmed their identities.

The BBC and most other international news organisations are barred from reporting inside Iran. (BBC)