By Bozorgmehr Sharafedin
LONDON (Reuters) - Iran's supreme leader accused the country's enemies on Tuesday of stirring unrest, as the death toll from anti-government demonstrations rose 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 were 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.
Nine people were killed in Isfahan province during protests on Monday night, including two members of the security forces, state television said.
Six protesters were killed during an attack on a police station in the town of Qahderijan. The governor of Falavarjan county said the protesters were armed.
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.
"Saudis will receive Iran’s unexpected response and they know how serious it can be," Shamkhani was quoted as saying by Tasnim news in an interview with Beirut-based Al Mayadeen TV.
Khamenei said on his website that he would address the nation about the events "when the time is right".
The head of Tehran's Revolutionary Court, Musa Ghazanfarabadi, said protesters would face harsh punishment.
Detainees would be put on trial soon and the ringleaders could be charged with "moharebeh" -- an Islamic term meaning warring against God -- which carries the death penalty, Ghazanfarabadi said.
Deputy Interior Minister Hossein Zolfaghari said 90 percent of the detainees were under 25-years-old, showing the younger generation's frustration at economic hardships and lack of social freedoms.
The demonstrations that broke out last week were initially focused on economic hardships and alleged corruption but turned into political rallies.
Anger was soon directed at the clerical leadership that has been in power since the 1979 revolution, including Ayatollah Khamenei, the ultimate authority in Iran’s system of dual clerical and republican rule.
Iran is an OPEC oil producer and regional power deeply involved in Syria and Iraq as part of a battle for influence with rival Saudi Arabia.
Many Iranians resent the foreign interventions and want their leaders to create jobs at home, where youth unemployment reached 29 percent last year.
President Hassan Rouhani refrained on Monday from accepting responsibility of problems raised by protesters and he blamed his predecessor and also Iran’s long-time adversary, the United States for the government's shortcomings.
Rouhani, seen as a pragmatist who is at odds with hardliners, said: "People on the streets do not ask for bread and water, but for more freedom," -- implying that the protesters were not targeting his government but the more rigid establishment.
Government spokesman Mohammad Baqer Nobakht told a news conference that both protesters and the security forces should follow the law.
"People have the right to protest but there is a difference between demonstration and riot ... Even those who are confronting the rioters should act within the framework of law," he said.
State television reported that protesters who set ablaze four mosques in villages in Savadkuh County in northern Iran on Monday had been arrested.
U.S. President Donald Trump, who has been tweeting daily in support of the protesters, 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!"
Iranian Foreign Ministry spokesman Bahram Qasemi said: "Instead of wasting his time by publishing offensive tweets to other nations, Trump should focus on internal affairs of America."
A U.S. official told Reuters U.S. intelligence officials think the protesters have little chance of toppling the government.
The Russian Foreign Ministry was quoted by the RIA news agency as saying external interference was destabilizing 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.
"We believe it is necessary to avoid violence and not succumb to provocations," the Turkish Foreign Ministry said, adding that it hoped foreign intervention would be avoided.
(Additional reporting by John Walcott in Washington, Vladimir Soldatkin in Moscow and Tulay Karadeniz in Ankara; Editing by Angus MacSwan and Robin Pomeroy)