Iraqi security forces fired live rounds on Thursday to break up protests held for a third day in Baghdad despite an open-ended curfew in effect since dawn.

Prime Minister Adel Abdel Mahdi ordered the ban on movements across the capital starting at 5:00 am (0200 GMT) to stem the popular demonstrations over widespread unemployment and state corruption.

The chaotic protests and ensuing clashes with riot police in Baghdad and several southern cities have left 12 protesters and one police officer dead. On Thursday morning, riot police fired in the air to disperse several dozen protesters gathered in the emblematic Tahrir Square in defiance of the curfew, an AFP photographer said.

"We slept here so the police don't take the place," one demonstrator told AFP before being pushed back en masse by police officers into adjacent sidestreets.

Tensions have been exacerbated by a near-total internet blackout, the closure of government offices in Baghdad and calls by firebrand cleric Moqtada al-Sadr for "a general strike."

Before dawn twin explosions hit the Green Zone, where some ministries and embassies are located and which was struck by two rockets last week, a security source in the area told AFP.

The apparent attack came hours after security forces sealed off the Green Zone "until further notice" just a few months after it was reopened to the public, fearing angry protesters would swarm it.

Overnight, two protesters were killed in the southern city of Kut after they tried to storm a local government office, medics and security sources told AFP.

Another two demonstrators died further south in Nasiriyah, which has so far seen the deadliest protests with a total of eight protesters and one police officer killed.

And two have died in clashes in Baghdad, bringing the nationwide toll in the protests since Tuesday to 13 dead and more than 400 wounded, according to health authorities.

Riot police in the capital have used water cannons, tear gas, rubber bullets and live rounds in an attempt to force protesters out of Tahrir and other areas in Baghdad. Into the night on Wednesday, marches from different parts of the capital attempted to converge on the square, whose name means "liberation" in Arabic.

But with internet access virtually shut, demonstrators have struggled to communicate with each other or post footage of the latest clashes.