Sadr, 47, who once led an anti-US militia and who has a large and dedicated following, is expected to have the key say in who will serve as the next prime minister, a post now held by Mustafa al-Kadhemi.

Sunday saw the swearing in of the 329 members of the unicameral parliament and the re-election of influential Sunni MP Mohammed al-Halbussi as speaker.

But a dispute broke out between deputies of the Shiite Coordination Framework coalition and their Sadrists rivals, several parliamentary sources said.

Iraq's post-election period has been marred by high tensions, violence and allegations of vote fraud.

MP Mahmud al-Mashhadani, the oldest member of parliament who chaired Sunday's session, was "hospitalised", public television said. Official news agency INA said he was in a "stable" condition.

A parliamentary source requesting anonymity said Mashhadani "fainted".

Videos filmed by MPs showed lawmakers fighting among themselves.

The session was suspended for more than an hour before resuming.

- 'Consensus' government? -

Sadr, who wears the black turban of a descendant of Islam's Prophet Mohammed, emerged as the big winner of the October 10 polls, which were held several months early as a concession to a pro-democracy protest movement.

His movement, which ran after he reversed an initial election boycott call, won 73 of the 329 seats.

Kurdish MP Muthana Amin said Sunday's session "began normally" but that the Coordination Framework claimed it was the largest alliance in parliament, with 88 seats.

Mashhadani "asked for the information to be verified, after which he was abused," Amin told AFP, without saying whether his hospitalisation was linked to the incident.

Within 30 days of its inaugural session, the parliament must elect the president of the republic.

The new president must then appoint a premier, who is chosen by the largest coalition and has 30 days to form a government.

Some experts and politicians expect a new governing team in place by March for the oil-rich but war-battered country of 40 million.

Sadr has repeatedly said he wants to break with the Iraqi political tradition of a "consensus" government to instead build a majority government.

That would mean securing a ruling majority that would appoint a premier and cabinet from within its ranks.

Forming a majority government would be "new" for Iraq, said political scientist Ihsan al-Shammari, noting that all Shiite parties had participated in previous administrations.

The post of premier historically goes to a Shiite, under Iraq's informal system of religious and ethnic quotas in place since the 2003 US-led invasion that toppled dictator Saddam Hussein.

- Instability and violence -

Sadr's pro-Iranian rivals in the Conquest Alliance, the political wing of the pro-Iran ex-paramilitary coalition Hashed al-Shaabi, won only 17 seats in the October election, compared to 48 in the previous parliament.

The Hashed's backers charged that the vote was marred by "fraud", but the courts rejected their appeal to have the election annulled.

Hashed supporters demonstrated in anger outside Baghdad's ultra-secure Green Zone, which houses parliament, other government buildings and the US embassy.

Kadhemi escaped unharmed when an unclaimed attack using armed drones targeted his residence on November 7.

Sadr has hinted that he prefers an alliance with Sunni groups Azm and Taqadom, and a Kurdish party, the KDP.

But the Conquest Alliance can count on its own allies including former prime minister Nuri al-Maliki, a figure close to Iran whose own group took 33 seats.