Seven security personnel were wounded in the attack, which took place amid a partial lockdown as parliament was convening. Security forces blocked bridges to the central Green Zone and imposed a curfew on buses, motorcycles and trucks.

Another rocket later fell near the Green Zone, where parliament and many government offices and foreign embassies are located, security sources said. There were no casualties.

Despite the tight restrictions, dozens of supporters of Shiite cleric Moqtada al-Sadr gathered in Baghdad's Tahrir Square, which lies outside the Green Zone, to protest against the parliamentary session.

A Reuters cameraman said around a dozen were seen throwing stones at security forces.

Sadr emerged as the biggest winner from an election last October but ordered his parliamentarians to withdraw after failing to form a coalition government after months of political deadlock. The Sadrists have called for fresh elections.

Halbousi, a Sunni politician who originally backed Sadr's efforts, has broken with him, arguing that efforts should continue on forming a government with other factions.

Earlier this year Halbousi's Taqaddum party and other Sunni and Kurdish factions supported Sadr's efforts to form a government that would exclude rival Iran-backed Shiite groups.

However, they did not follow suit when Sadr withdrew from parliament, and have instead considered entering a ruling alliance with the Iran-backed parties, according to officials on all sides of the political divide.

A large majority of parliamentarians voted on Wednesday against Halbousi's resignation, effectively endorsing his continuation in office.

The leader of a militia loyal to Sadr condemned Wednesday's rocket attack. "We condemn and denounce the shelling of the Green Zone today and we stress the constitutional right to protest," Peace Brigades commander Abu Mustafa al-Hamidawi said.