Iran's Supreme leader Ayatollah Ali Khamenei said the enemy had been "repelled" in the country where dozens are thought to have died in violent protests sparked by a petrol price hike.

Demonstrations broke out in the sanctions-hit Islamic republic on Friday after it was announced the price of petrol would be raised by as much as 200 per cent.

"We have pushed back the enemy in the military arena. We have pushed back the enemy in the political war," Khamenei said in a speech aired on state television late on Tuesday.

"We have repelled the enemy in the arena of security warfare [...] in recent days," he said, adding that the unrest had not been the result of a popular movement.

"The recent actions were security issues, not from the people," he said.

"We have repelled the enemy."

State television, which rarely shows any signs of dissent in Iran, has aired footage of masked young men appearing to clash with security forces.

The United Nations called for restraint on Tuesday and voiced alarm at reports dozens may have been killed in the violence.

London-based rights group Amnesty International said more than 100 demonstrators were believed to have been killed across Iran since security forces were ordered to "crush" the protests.

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.