The Iraqi government's two main supporters agreed early Wednesday to drop their support for embattled premier Adel Abdel Mahdi, overcoming their rivalry to resolve a political crisis sparked by widespread demonstrations.

The surprise rapprochement came as tens of thousands flocked to the capital's Tahrir Square to demand the government quit, marking the largest rally yet since protests erupted on October 1.

Populist cleric Moqtada al-Sadr, who leads parliament's largest bloc, has spearheaded calls this week for Abdel Mahdi's resignation and for early parliamentary elections.

But Hadi al-Ameri, the chief of second-biggest bloc Fatah and a leading commander in its armed Hashed al-Shaabi branch, had thrown his weight behind the government.

In an unexpected move, Ameri announced overnight that he and Sadr would "work together to achieve the people's demands."

He had accepted Sadr's public invitation, issued hours earlier, to coordinate a no-confidence vote in Abdel Mahdi, whom he dramatically called to "Get out!" in a tweet.

Sadr and Ameri formed a tenuous alliance to bring Abdel Mahdi to power last year but have been increasingly at odds in recent months.

Their partnership was further strained when several Hashed offices were torched in southern Iraq last week during protests.

Parliament is set to meet for a third consecutive day on Wednesday, after they agreed to explore early polls and constitutional amendments and demanded Abdel Mahdi come in for questioning "immediately."

The premier has so far ignored those calls, and on Tuesday dismissed demands for early elections, instead challenging Sadr to find a political solution.

If a new government was formed, Abdel Mahdi wrote, it could begin work within "days, if not hours."

Sadr, who had just returned to his hometown of Najaf from neighboring Iran, appeared to take up the challenge.

As the politics played out online, the public demonstrations continued late into the night on Tuesday in Tahrir Square.

Blaring horns, fireworks and loud Iraqi music filled the plaza, Baghdad's focal point for demonstrations.

"They said we wouldn't be able to do anything. But even if we change one name, now we have a voice," said Youssef, a 33-year-old who was spending his sixth straight night in the square.

Protesters have defied army orders to clear the streets between midnight and 6:00 am local time, occupying the square and multi-story buildings there.

Late Tuesday, security forces unleashed volleys of tear gas at crowds massing on a key bridge linking Tahrir to the Green Zone, which hosts government offices and foreign embassies.

But the crowds seemed unfazed. "We've started fighting over who can kick back the tear gas canister first," Youssef joked.

Rallies in Baghdad and across the south have swelled in recent days, defying curfews. Threats of arrest and violence have left more than 240 people dead and more than 8,000 wounded.

(With input from AFP)