At least 23 anti-government protesters were shot dead by unknown forces on Friday, according to an estimate by Reuters, citing police and medical sources.
The attack came mere hours after the United States sanctioned three Iranian-backed Iraqi paramilitary leaders, accusing them of being responsible for deadly attacks against anti-government protesters.
"Three of today's designees...were directed by the Iranian regime when they or the armed group they lead committed serious human rights abuses," said David Schenker, the US' Assistant Secretary for Near Eastern Affairs.
One of them is the head of Asaib Ahl al-Haq, a powerful Iran-backed militia accused of being behind sniping attacks on protesters.
The U.S. Treasury sanctioned leader Qais al-Khazali, his brother Laith al-Khazali, who is a commander in the group, and Husain Falih Aziz al-Lami.
At least 400 people have died since the uprising shook Iraq on October 1.
Thousands of Iraqis have taken to the streets in Baghdad and the predominantly Shiite southern Iraq decrying corruption, poor services, lack of jobs and calling for an end to the political system that was imposed after the 2003 US invasion.
Security forces have regularly used live rounds and tear gas to disperse the demonstrations, leading to heavy casualties.
Anti-government activists have sought to blame supporters of Iran-backed Iraqi militias, which have staged similar attacks against protester sit-ins in the capital and the country's southern cities.
A string of mysterious knife attacks against anti-government protesters also occurred on Thursday in the square, after supporters of the Iran-backed militias attempted their own rival demonstration before withdrawing.