Demonstrations first erupted on October 1 over corruption and unemployment and have since ballooned, with protesters now insisting on a government overhaul.

Their demands have been backed by populist cleric Moqtada Sadr, whose parliamentary bloc Saeroon has been gathering support to force the premier to come in for questioning.

Abdel Mahdi has so far resisted, saying one condition for his appearance would be that the session be televised.

Lawmakers met on Thursday for a fourth consecutive day and agreed to broadcast any session live, with Saeroon's MPs chanting "Adel must come! Adel must come!"

President Barham Saleh also held closed-door talks on Thursday with Iraq's main political figures, a source in the president's office said.

"Things are heading towards a resolution," another senior Iraqi official told AFP.

The president was expected to speak on Thursday in a much-awaited speech, his first in weeks.

The United Nations' top representative in Iraq, Jeanine Hennis-Plasschaert, called for a national dialogue to draw a roadmap out of the crisis.

"Today Iraq stands at a crossroads. Progress through dialogue, or divisive inaction," she said.

"Full access to all information, facts and figures will prove key. Window dressing will only feed anger and resentment," she added.

- 'Let them leave' -

Across the Shiite-majority south, demonstrators came out in force on Thursday despite efforts to quell them with curfews, tear gas or live fire.

The southern city of Diwaniyah saw its largest rallies yet: students, teachers, farmers and health workers hit the streets as government offices remained closed.

In Basra, demonstrators cut off a main road leading to the Umm Qasr port, its authorities said, one of the main import zones for food and other supplies into Iraq.

In Baghdad, crowds occupied the emblematic Tahrir (Liberation) Square for the eighth consecutive day.

"We're tired of the whole situation over the past 16 years. The country went from bad to worse," said Salwa Mezher, a middle-aged woman protesting with the Iraqi flag around her shoulders.

Since the US-led invasion that toppled dictator Saddam Hussein in 2003, Iraq's political system has been gripped by clientelism, corruption and sectarianism.

Getting a job in government, the country's biggest employer, is often secured with bribes or connections.

One in five Iraqis live below the poverty line and youth unemployment stands at 25 percent, despite the vast oil wealth of OPEC's second-largest crude producer.

That inequality has been a rallying cry for protesters.

"Our problem isn't just with Adel Abdel Mahdi, it's with them all," said Mezher, before adding a refrain popularised in this month's protests: "Weed them all out!"

The protests are unique in Iraq's recent history for their fury at the entire leadership, even typically revered clerics.

"We don't want them, so let them leave. We also don't want the clerics -- they have no business in politics," said Hoda, a 59-year-old in a headscarf and sunglasses.

Demonstrators packed onto two bridges leading to the Green Zone, where government buildings and foreign embassies are based, setting up barricades to face off against riot police trying to hold them off with tear gas.

Late on Wednesday, a rocket attack hit a checkpoint near the US embassy, killing one Iraqi military member and wounding others, security sources told AFP.

- 'Trapped, dependent' -

At least 257 people have died and 10,000 have been wounded since protests broke out on October 1, with 100 people losing their lives in the last week, the Iraqi Human Rights Commission said.

Abdel Mahdi, 77, came to power a year ago through a tenuous partnership between Sadr and Hadi al-Ameri, a member of the powerful Hashed al-Shaabi paramilitary force.

That alliance has frayed in recent months, and Sadr threw his weight behind the protests while Ameri and his allies backed the government.

A rapprochement built on Abdel Mahdi's ouster appeared close on Tuesday night, but disagreements over who could replace him seemed to have slowed down the process.

Any candidate would have to be "presentable to the parliament and accepted by the streets," said Maria Fantappie, an expert at the International Crisis Group.

"A consensus candidate with a technocratic background? We know the ending of that story," said Fantappie, referring to Abdel Mahdi's rocky tenure.

"He will once again be trapped and dependent on these two blocs, and it will bring the same kind of discontent in the streets."