Iran warned on Sunday that protesters will "pay the price" after a third night of unrest saw mass demonstrations across the country, two people killed and dozens arrested.

Videos on social media showed thousands marching across the country overnight in the biggest test for the Islamic republic since mass protests in 2009.

They showed demonstrations in Mashhad, Isfahan and many smaller cities but travel restrictions and limited coverage by official media made it difficult to confirm reports.

State media began to show footage of the protests on Sunday, focusing on attacks by young men against banks and vehicles, an attack on a town hall in Tehran, and images of a man burning the Iranian flag.

"Those who damage public property, disrupt order and break the law must be responsible for their behaviour and pay the price," Interior Minister Abdolrahman Rahmani Fazli said on state television.

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