The UN on Tuesday said it was alarmed at reports that scores have been killed in Iranian demonstrations against rises in petrol prices, as Tehran said it would unblock the internet only after calm has been restored.
Amnesty International said more than 100 demonstrators were believed to have been killed across Iran in five days since security forces were ordered to crush the protests. Tehran has denounced the demonstrators as “rioters”.
Iran's economy has been hampered since May last year when the US withdrew from a 2015 nuclear agreement between Tehran and world powers and reimposed sanctions.
Iran's decision on Friday to increase fuel prices by 50 per cent sparked the protests in which authorities have confirmed at least five deaths, including three security personnel who officials say were stabbed by protesters.
President Hassan Rouhani said the government was acting in the public interest, and that the money raised would be distributed to the country's neediest citizens.
The UN rights office said it was alarmed by reports that live ammunition was being used against protesters and had caused a "significant number of deaths across the country".
Spokesman Rupert Colville said that casualty details were hard to verify, partially because of a three-day internet shutdown.
"Iranian media and other sources suggest dozens of people may have been killed and many people injured during protests in at least eight different provinces, with over 1,000 protesters arrested," Mr Colville said in Geneva.
"We urge the Iranian authorities and security forces to avoid the use of force to disperse peaceful assemblies."
He called on protesters to demonstrate peacefully and not resort to violence.
Two petrol stations in Tehran were gutted by fire and there was damage to other buildings including a police station, AFP reported.
State television showed footage of rallies against "rioting" held in the north-western city of Tariz and in Shahr-e Kord, central Iran.
"Protesting is the people's right, rioting is the work of enemies," they chanted in Tabriz, the Fars news agency reported.
When the protests began on Friday, drivers stopped on major roads in Tehran to block traffic.
The protests soon turned violent and spread to more than 40 cities and towns, with banks, petrol stations and other property set ablaze and shops looted.
"At least 106 protesters in 21 cities have been killed, according to credible reports," Amnesty said. "The real death toll may be much higher, with some reports suggesting as many as 200 have been killed."
Unofficial reports say 3,000 people have been injured in the protests over the past five days.
"Authorities must end this brutal and deadly crackdown immediately," Amnesty's Philip Luther said.
Mr Luther said the figures were based on verified video footage, witnesses and information from rights activists outside Iran.
Amnesty also urged Iran's government to unblock the internet, which it said was restricting information about the crackdown to the rest of the world.
State television, which rarely shows any signs of dissent, broadcast footage of masked men clashing with security forces.
In a video aired Monday night, a man fires an assault rifle as others hurl stones at security forces in the western city of Andimeshk.
Also on Monday, assailants wielding knives and machetes ambushed and killed three security personnel west of Tehran, local news agencies reported.
One of those killed was named as Morteza Ebrahimi, a commander of the Islamic Revolutionary Guard Corps, while the others were members of the pro-regime Basij militia, a volunteer force.
It is the worst violence since at least 25 people were killed in protests over economic hardship that started in Iran's second city Mashhad in December 2017, before spreading to other towns.