Iran's supreme leader is accusing the country's enemies of stirring unrest, as the death toll from anti-government demonstrations rises to 21.
On the sixth day of protests, riot police were out in force in several cities, footage on social media showed, as security forces struggle to contain the boldest challenge to Iran's clerical leadership since unrest in 2009.
More than 450 protesters have been arrested in the capital in the last three days, Tehran's deputy provincial governor said, and hundreds of others were detained around the country, with judicial officials saying they would be severely punished.
In his first reaction to the unrest, Ayatollah Ali Khamenei said: "In recent days, enemies of Iran used different tools including cash, weapons, politics and intelligence apparatus to create troubles for the Islamic Republic."
He did not mention any enemies by name but Ali Shamkhani, secretary of the Supreme National Security Council, said the United States, Britain and Saudi Arabia were behind the riots.
The head of Tehran's Revolutionary Court, Musa Ghazanfarabadi, said protesters would face harsh punishment, including the death penalty.
Deputy Interior Minister Hossein Zolfaghari said 90 per cent of the detainees are under 25-years-old, showing the younger generation's frustration at economic hardships and lack of social freedoms.
The demonstrations initially focused on economic hardships and alleged corruption but turned into political rallies.
President Hassan Rouhani refrained on Monday from accepting responsibility of problems raised by protesters and blamed his predecessor and also Iran's long- time adversary, the United States for the government's shortcomings.
President Donald Trump, who has been tweeting support of the protesters daily, said on Tuesday: "The people of Iran are finally acting against the brutal and corrupt Iranian regime ... The people have little food, big inflation and no human rights. The U.S. is watching!"
The Russian Foreign Ministry was quoted by the RIA news agency as saying external interference was destabilising the situation and calling it "unacceptable".
Iran and Russia are the main allies of Syrian President Bashar al-Assad, while the United States, Saudi Arabia and Turkey support rebel groups.
Turkey called for restraint.