There were reports of protests breaking out in Iran for the fifth day on Tuesday, despite a statement from President Hassan Rouhani that the unrest "is nothing".
Anti-government chanting and burning cars were reported as night fell in Tehran, while police said an officer was killed in a central city, BBC reported.
President Hassan Rouhani said protests were an "opportunity, not a threat" but vowed to crack down on "lawbreakers".
The US, meanwhile, stepped up support for the protesters' "bold resistance".
The protests began last Thursday in the city of Mashhad, initially against price rises and corruption but now with wider anti-government sentiment.
The latest reports speak of a heavy police presence in the capital. The Mehr news agency reported a taxi being set alight. Police had used tear gas and water cannon the previous evening to quell a rally in Tehran's Engheleb Square.
A police spokesman as quoted as saying that shots had been fired at police in Najafabad, near Isfahan in central Iran, killing one officer and wounding three.
Social media postings spoke of fresh protests in Birjand in the east, Kermanshah in the west and Shadegan in the far south-west.
In a statement on the presidency website, Rouhani sought to play down the violence.
He said: "This is nothing. Criticism and protest are an opportunity not a threat."
But he also vowed to act against "rioters and lawbreakers".
Iran's Revolutionary Guards Corps (IRGC) has taken a tough line, warning anti-government protesters they will face the nation's "iron fist" if political unrest continues.
The IRGC is a powerful force with ties to the country's supreme leader, and is dedicated to preserving the country's Islamic system.
Judiciary Chief Ayatollah Sadeq Amoli-Larijani on Monday called for a crackdown on "rioters" and "vandals".
"Some individuals are exploiting the situation. This is wrong," he said.
Up to 400 people are reported to have been arrested in recent days.
At least 12 people have been killed in recent rallies across Iran in protest against the regime of President Hassan Rouhani.
--IANS
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