Baghdad (CNN) -- Iraqi soldiers fired Friday on Sunni anti-government protesters demanding the Shiite prime minister step down, health officials said.
Provincial health officials said that at least four people were killed and 42 wounded in the shootings in Falluja in western Anbar province, a Sunni stronghold where protesters have been denouncing what they call second-class treatment by the government.
The officials backed off earlier reports that at least five people were killed, citing the chaos of scene.
Witnesses told CNN that Iraqi soldiers opened fire after they ordered the demonstrators to stop filming dozens of Iraqi security forces on the rooftops surrounding al-Etisam Square.
But other witnesses said that Iraqi soldiers fired when protesters started throwing objects at them.
The soldiers continued firing live bullets for several minutes.
Angry protesters chanted: "Those are al-Maliki's forces, those are al-Maliki's militias," referring to Prime Minister Nuri al-Maliki.
Iraqi authorities imposed curfew in Falluja on Friday until further notice.
Religious leaders in Falluja have called for Iraqi forces to leave the city and for a funeral procession Saturday for those killed in the incident.
"We have been demonstrating peacefully. But now we warn al-Maliki that the blood of Falluja martyrs will not be shed in vain," protester Abdul Wahed Ammar said.
Al-Maliki on Friday said the violence in Falluja doesn't surprise his administration. He cited "conspiracies" plotted by regional intelligence services, vestiges of the old regime, al Qaeda and those with sectarian agendas.
The government, he said, has warned before about "those who have hostile agendas against Iraq, its political process and its democracy." He said dangers are "increasing day after day in an attempt to blow up the security situation in the country and drag the armed forces into confrontations."
The minister of defense ordered an investigation into the shootings, Anbar Satellite TV said. Iraq's government spokesman did not immediately respond to a CNN request for comment.
The protest in Falluja, dubbed "Friday of No Retreat" by organizers, is the latest in a series held in predominantly Sunni regions of Iraq. They have been countered by mostly Shiite, pro-government demonstrations, raising fears that the sectarian division could bring violence in the streets.
In another development , Iraqi soldiers withdrew from several security posts in and around the city and went back to their main military headquarters in Falluja, according to police officials in the city.
The protests have grown in recent weeks. They began in late December when Sunni demonstrators took to the streets in Anbar province, which borders Jordan and Syria, to protest al-Maliki's order to arrest the bodyguards of Finance Minister Rafaie Esawi, a Sunni.
The arrest of Esawi's bodyguards came just hours after President Jalal Talabani, a Kurd who is widely viewed as a stabilizing political force in Iraq, left the country to undergo treatment for cancer.
The protesters also are demanding the release of detainees they said were held without charges, calling the government corrupt and accusing it of unfairly targeting Iraq's Sunni people.
Iraq's Arab Sunnis and Kurds have accused al-Maliki and his Shiite political party of working to consolidate power in Iraq by cutting them out of the political process, an allegation that comes as U.S. lawmakers raise concerns about Iraq strengthening its ties with Shiite-dominated Iran.
Sunnis make up about 20% of Iraq's estimated population of more than 27 million, whereas about 60% to 65% are Shiite.
Since the fall of Saddam Hussein's Sunni-dominated regime in 2003, Sunnis in Iraq have been largely disaffected. The gulf was widened in 2005 when Sunnis boycotted the country's election, opening the way to a heavily dominated Shiite government.
The sectarian divisions translated into violence in the streets in 2006 and 2007, with fighting that nearly ripped the country apart.
CNN's Chelsea J. Carter contributed to this report.