Allawi's nomination on February 1 has so far failed to quell the months-long rallies sweeping Baghdad and the mainly-Shiite south, where young demonstrators have demanded nothing short of a total government overhaul.

Most young protesters have rejected Allawi as too close to the ruling elite but powerful cleric Moqtada Sadr, who has backed the rallies, welcomed his appointment on Saturday.

Sadr, however, urged his followers to stay in the streets, creating a rift within protest squares.

Late Sunday, anti-government demonstrators opposed to Allawi's nomination started to cluster their tents closer together in Baghdad's Tahrir Square, activists at the heart of the protest movement said.

"They're split into two parts now, and there are plenty of people on both sides. I'm worried about a clash," one long-time protester in Tahrir told AFP.

Earlier, dozens of Sadrists had stormed a key building in Tahrir that had been occupied for months by protesters, driving out activists and removing banners listing their demands.

In the oil-rich southern port city of Basra, university students relocated their tents overnight to move away from those occupied by Sadr supporters, an AFP reporter said.

"If Sadrists come to the protest square, don't come into contact with them, don't make problems," one organiser there called out over a loudspeaker.

Sadr, a 45-year-old enigmatic figure with a cult-like following across Baghdad, backed the protests when they first broke out in October but has rethought his support multiple times since.

After urging his supporters back to the streets on Friday, he endorsed Allawi and condemned student sit-ins and road closures -- the two main tactics used by most protesters.

On Monday, Sadrists -- identifiable by their blue caps -- deployed around schools and government offices in Kut and Hillah, south of the capital, to ensure they would fully reopen after weeks of intermittent closures due to the rallies, AFP correspondents said.