Sadr, an enigmatic militiaman-turned-politician, backed the anti-government rallies when they erupted in October but has split with other demonstrators over the nomination of Mohammad Allawi as prime minister.

The cleric endorsed Allawi while other protesters rejected him, charging he is too close to the ruling elite they have been demonstrating against for four months.

In the southern city of Diwaniyah on Tuesday, the rift escalated into a fistfight between young anti-regime demonstrators and Sadr backers, who are recognisable by their signature blue head caps, an AFP correspondent said.

Police intervened to separate the two camps but the young protesters broke into chants against Sadr, Iraqi authorities as well as Iran, accused by demonstrators of backing the government's crackdown against them.

Sadr took to Twitter on Tuesday afternoon in an apparent attempt to calm the tensions.

"The 'blue hats' have a duty to peacefully secure schools and service centres, not to defend me or suppress the voices that chant against me," he said.

Tensions have been high in protest squares in recent days between youths furious at Allawi's nomination and Sadrists.

On Monday, a demonstrator was stabbed to death and three others wounded after men in blue caps attacked an anti-regime rally, medics and security sources said.

Sadr had called for his supporters to help reopen schools, roads and public offices shuttered by months of sit-ins and the interior ministry on Monday followed up with an order for reinforcements to deploy at schools.

Security forces could be seen outside schools in Diwaniyah as a few students trickled in on Tuesday.

Hundreds of students refused to go back to class, however, marching through the main anti-government protest camp with Iraqi flags and a banner that read, "Protest March for Diwaniyah High Schools."

In Nasiriyah, too, all schools had reopened after police deployed, according to the education directorate's press chief Halim al-Hossayni.

But students took to the streets there as well to insist on keeping up their protests.

"We're determined to pursue our peaceful movement in Habbubi square, because we want a homeland free of corruption and sectarian people," said student Hamad Ali.

Allawi, 65, was nominated on February 1 after two months of political stalemate over who would replace ex-premier Adel Abdel Mahdi, who resigned in December.