UN Secretary-General Antonio Guterres on Tuesday expressed his "regret" at the rising death toll in anti-government protests in Iran, and called on the Islamic Republic to respect the rights of peaceful protesters.

Tehran said 21 people had died in clashes across the country as thousands of people took to the streets. Police have arrested hundreds of demonstrators.

"We regret the reported loss of life and hope further violence will be avoided. We expect that the rights to peaceful assembly and expression of the Iranian people will be respected," Guterres spokesman Farhan Aziz Haq said on behalf of the secretary-general.

Iran's Supreme leader Ayatollah Ali Khamenei had blamed the country's "enemies" earlier Tuesday for almost six days of deadly unrest that have shaken the conservative country.

"The enemies have united and are using all their means, money, weapons, policies and security services to create problems for the Islamic regime," Khamenei said.

A fifth night of unrest Monday to Tuesday saw six protesters killed during an attack on a police station in Qahderijan in the central province of Isfahan, state TV said, bringing the death toll to 21.

Source: AFP