Iran’s supreme leader accused the country’s enemies on Tuesday of being behind days of street unrest, as the death toll from anti-government demonstrations rose to 21.

Riot police were out in force in several cities on the sixth day of protests, which spread to the northwestern city of Tabriz, according to social media posts, as security forces scrambled to contain the boldest challenge to Iran’s leadership since 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. A judicial official said some could face the death penalty.

In the southwestern city of Ahvaz, police fired tear gas to disperse hundreds of demonstrators.

“Police shot tear gas and later went around asking shops on two main streets to close, to completely clear the area,” a resident told Reuters by telephone.

Nine people were killed in Isfahan province during protests on Monday night, including two members of the security forces, state TV said. Six protesters were killed during an attack on a police station in the town of Qahderijan in that province. The local governor said the protesters were armed.

Supreme leader Ayatollah Ali Khamenei accused Iran’s foes of fomenting the unrest, which began as protests about economic hardship and corruption but have grown into political rallies, some of which have criticized him by name.

“In recent days, enemies of Iran used different tools including cash, weapons, politics and intelligence apparatus to create troubles for the Islamic Republic,” he said, in his first reaction to the unrest.

He did not name the enemies 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 by Tasnim news as saying 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 United States rejected the suggestion that the protests were the work of foreign powers.

“We all know that’s complete nonsense,” said Nikki Haley, the US ambassador to the United Nations.

“The demonstrations are completely spontaneous. They are virtually in every city in Iran. This is the precise picture of a long oppressed people rising up against their dictators.”