The death toll after armed men attacked anti-government demonstrators in central Baghdad rose to at least 24 Saturday, officials said.
Police and medical sources in Iraq's capital said more than 127 others were injured by gunfire and stabbings in the attack, which began late Friday. Three police officers were among those killed.
The attack ranks among the most deadly in Baghdad since thousands of Iraqis launched street demonstrations October 1. Protesters have been demanding broad political reforms and a halt to Iran's influence in Iraqi affairs.
Heavily armed, masked gunmen, whom security forces said they could not identify, targeted protesters near Khilani Square in downtown Baghdad. Security sources said the gunmen entered the main street leading to the square in a convoy, firing weapons during their approach.
They roamed the area near the square, but Iraqi security personnel forced them to turn around at a checkpoint. Gunfire, however, continued into early Saturday.
In a statement Saturday, Jeanine Hennis-Plasschaert, special representative of the U.N. secretary-general for Iraq, condemned "in the strongest terms the shooting of unarmed protesters in central Baghdad on Friday night." She called the attack "nothing less than an atrocity against the people of Iraq. The perpetrators must be identified and brought to justice without delay."
She urged the Iraqi armed forces "to spare no effort to protect the peaceful protesters from violence by armed elements operating outside state control."
The incidents came one week after Prime Minister Adel Abdul-Mahdi said he would resign after two months of anti-government protests.
Friday's attack occurred hours after the U.S. placed sanctions on three Iran-backed Iraqi militia leaders whom Washington accuses of directing the killing of the protesters.
Also, the head of Iraq's Sairoon political party said a drone attacked his home in the holy city of Najaf early Saturday, according to statements from the party.
Muqtada al-Sadr, an influential Shiite cleric who has supported the anti-government protesters, was not home at the time of the attack.
The head of the party's political committee denounced the drone attack and called for an emergency parliamentary session to discuss the violence.
Nearly 450 people have died and about 20,000 others have been wounded since the demonstrations erupted in October in Baghdad and in Shiite-majority areas in southern Iraq.