TEHRAN — Three security personnel have been killed by "rioters" in Iran, the latest deaths in protests that showed no sign of subsiding on Tuesday, despite a wave of arrests and an internet shutdown.

A shock decision to impose petrol price hikes sparked the nationwide demonstrations in which officials say at least five people have been killed.

The United Nations expressed alarm on Tuesday "that the use of live ammunition has allegedly caused a significant number of deaths across the country".

The situation in the streets remains unclear largely due to the government-imposed internet outage now in its third day.

AFP journalists saw two petrol stations in Tehran gutted by fire and damage to infrastructure, including a police station.

But they were prevented from filming as hundreds of riot police guarded squares in the capital with armored vehicles and water cannon.

Rallies against "rioting" were held in the northwestern city of Tabriz and Shahr-e Qods, west of Tehran, Iranian news agencies reported.

"Protesting is the people's right, rioting is the work of enemies," they chanted in Tabriz, according to Fars news agency.

When the demonstrations began on Friday, drivers stopped on major thoroughfares 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 public property set ablaze and shops looted.

The demonstrations erupted after it was announced the price of petrol would be raised by 50 percent for the first 60 liters purchased over a month and 200 percent for any extra fuel after that.

Iran's economy has been battered since May last year when the United States unilaterally withdrew from a 2015 nuclear agreement and reimposed crippling sanctions.

Footage of masked young men clashing with security forces has been broadcast on state television, which rarely shows any signs of dissent.

In a video aired Monday night, a man can be seen firing what appears to be an assault rifle as others hurl stones apparently at security forces in the western city of Andimeshk.

In the latest bloodshed, assailants wielding knives and machetes ambushed and killed three security personnel west of Tehran, the ISNA and Fars news agencies reported late Monday.

One of the three was identified as Morteza Ebrahimi, a commander of the Islamic Revolutionary Guard Corps and father of a newborn child, according to Fars.

The other two were Majid Sheikhi, 22, and Mostafa Rezaie, 33. Both served in the Basij militia, a volunteer force loyal to the establishment.

The overall toll — which also includes a civilian and policeman — is expected to rise, with unofficial reports saying that at least six other people have been killed.

It is the worst violence since at least 25 lives were lost in protests over economic hardship that started in Iran's second city Mashhad in December 2017 before spreading to other urban centres.

In response to the violence, the authorities say they have arrested hundreds of people.

Iran said on Tuesday the internet will only be unblocked when authorities are sure it will not be misused.

"The internet will come back gradually in some provinces where there are assurances the internet will not be abused," government spokesman Ali Rabiei said.

The outage has stemmed the flow of videos shared on social media of protests or associated acts of violence.

Netblocks, a website that monitors global net shutdowns, said internet connectivity in Iran was at four percent on Tuesday compared with normal levels.

"Sixty-five hours after #Iran implemented a near-total internet shutdown, some of the last remaining networks are now being cut," it tweeted.

Iran announced the decision to impose petrol price hikes and rationing at midnight Thursday-Friday, saying it was aimed at helping the needy.

The plan, agreed by the president, parliament speaker and judiciary chief, comes at a sensitive time ahead of February parliamentary elections.

It has received the public support of Iran's supreme leader Ayatollah Ali Khamenei.

President Hassan Rohani has defended the price hike, saying the proceeds will go to 60 million Iranians.

The US has condemned Iran for using "lethal force".

Iran hit back, slamming Secretary of State Mike Pompeo after he tweeted "the United States is with you" in response to the demonstrations.

In Geneva, the UN rights office expressed alarm over the alleged use of live ammunition against protesters.

"We urge the Iranian authorities and security forces to avoid the use of force to disperse peaceful assemblies," said spokesman Rupert Colville.

"We also urge protesters to carry out demonstrations peacefully, without resorting to physical violence or destruction of property."

Iran's judiciary spokesman, Gholamhossein Esmaili, warned the authorities would deal firmly with those who endanger security and carry out arson attacks.

He also called on citizens to inform on "seditionists" who have committed acts of violence. — AFP