Dubai - Iran's judiciary should take tough measures against protesters and anyone who thinks the country's rulers will fall is dreaming, a senior cleric said on Friday.

The Islamic Republic has been gripped by demonstrations that erupted after the death in police custody of 22-year-old Mahsa Amini last month.

"The judiciary should deal with the rioters - who betrayed the nation and poured water into the enemy's watermill - in such a way that others don't again fancy to riot," hardline cleric Ahmad Khatami said in a Friday prayers sermon in Tehran, Iranian media reported.

"They have told deceived kids if they stay in the streets for a week the regime will fall. Dream on!"

Iran has blamed "thugs" linked to "foreign enemies" for the unrest.

The nationwide protests have turned into one of the boldest challenges to Iran's clerical rulers since the 1979 revolution. Protesters have called for the downfall of the Islamic Republic, although the protests do not seem close to toppling the system.

On Friday, police arrested at least 57 "rioters" after protesters threw rocks and attacked banks in the city of Zahedan, the provincial police chief, Ahmad Taheri, was quoted as saying by the official news agency IRNA.

Zahedan is the capital of restive southeastern Sistan-Baluchistan province which is the home of Iran's Baluchi minority. Amnesty International has said security forces killed at least 66 people in a violent crackdown after Friday prayers in Zahedan on Sept. 30.

Videos posted on social media purported to show demonstrators in Zahedan chanting "Death to the dictator", a reference to Supreme Leader Ayatollah Ali Khamenei, and "Death to Basijis", in reference to the Basij militia which has been widely used to crack down on protests.

Reuters could not independently verify the videos.

As protests continued in several cities, the activist website 1500tasvir posted a video it said showed a demonstration in the central city of Isfahan and footage purporting to show protesters lighting fires on streets of the northwestern city of Mahabad late on Thursday.

Videos of protests have been delayed because of internet restrictions imposed in Iran by authorities, activists say.

The activist news agency HRANA said in a posting that 244 protesters had been killed in the unrest, including 32 minors.

It said 28 members of the security forces were killed and more than 12,500 people had been arrested as of Thursday in protests in 114 cities and towns and some 81 universities.

(Reporting by Michael Georgy; Editing by Angus MacSwan)