Gunmen in cars opened fire on Friday in Baghdad's Khilani Square, leaving at least 15 people dead and 60 wounded, Iraqi security and medical officials said.
At least two of the dead were policemen.
Protesters fearing for their lives ran from the plaza to nearby Tahrir Square and mosques to take cover. It wasn't immediately clear who did the shooting.
Eyewitnesses said several demonstrators were killed late on Friday when unidentified men attacked and seized a building near the main protest camp in Iraq's capital.
Protesters had feared possible violence since Thursday, when thousands of supporters of the Hashed Al-Shaabi security force descended on Tahrir Square.
Protesters ran from the plaza to nearby Tahrir Square and mosques to take cover. Protesters are occupying part of Jumhuriya, Sinak and Ahar bridges in a standoff with security forces. All bridges lead to or near the heavily-fortified Green Zone, the seat of Iraq’s government.
The attacks come one day after a string of suspicious stabbing incidents left at least 13 wounded in Baghdad’s Tahrir Square, the epicenter of Iraq’s leaderless protest movement.
The attacks by unknown perpetrators occurred as demonstrators supporting political parties and Iran-backed militias withdrew from the Square on Thursday evening.