Protesters have occupied Baghdad's Tahrir (Liberation) Square for more than three weeks, demanding the overhaul of a ruling system they see as corrupt and unjust.

On Friday night, at least one person was killed and more than a dozen wounded in Tahrir when explosives beneath a parked car detonated, Iraq's state security forces said.

The protest movement had spilled over onto four bridges crossing the river Tigris that link east Baghdad to the city's west where government buildings and foreign embassies are based.

Security forces retook three of those bridges and nearby districts more than two weeks ago, pinning the protesters back in Tahrir and on Al-Jumhuriyah bridge with volleys of tear gas, live ammunition and even machine-gun fire.

On Saturday morning, Iraqi units pulled back from some of those areas and crowds of protesters chased them down, resuming their sit-in at the mouth of Al-Sinek bridge.

"The security forces withdrew to another concrete barrier on Al-Sinek," one protester told AFP.

An elderly woman, who had travelled from the southern port city of Basra to join the rallies, cheered in support.

"You didn't just lose us, you lost all of Iraq!" she said, addressing Prime Minister Adel Abdel Mahdi.

"Get out, get out, there's no place for you here. Tonight, we'll be in the Green Zone," she said, referring to the area across the river housing Abdel Mahdi's office, parliament, the US embassy and other key buildings.

Dozens climbed up into a large parking complex near the bridge, unfurling a sign in support of demonstrators in nearby Tahrir.

The morning after Friday's blast, small clusters of men deployed around the square to search all those entering it.

"We had a security breach yesterday and this explosion happened," said Abu Karrar al-Basrawi, a middle-aged man from Basra volunteering for the search.

"But we've multiplied our checkpoints so it doesn't happen again," he told AFP.