(CNN) At least 12 demonstrators were killed and 22 others wounded Friday when gunmen in several vehicles opened fire on a protest site in central Baghdad, medical sources and activists told CNN, marking one of the most violent days in the Iraqi capital for weeks.

As thousands of anti-government demonstrators rallied in Baghdad's Tahrir Square -- the epicenter of the protest movement -- the gunmen seized a nearby parking garage that is used by activists.

The gunmen have not yet been identified.

An ambulance arrives in Tahrir Square after gunmen opened fire on anti-government protesters.

Hundreds of members of pro-government militias have descended on Tahrir Square in a show of force over recent days, raising fears over further unrest, several activists told CNN.

During his weekly Friday sermon, Iraq's top Shiite cleric warned against meddling, saying that the country's new premier must be chosen "without any foreign interference." The clock is ticking for lawmakers to replace Prime Minister Adil Abdul Mahdi , who announced his resignation last week, calling on Iraq's government to "preserve the blood of its people" and "avoid slipping into a cycle of violence, chaos and devastation."

Read More