More than 1,000 Shiite Muslim demonstrators clashed Wednesday night with supporters of influential Shiite cleric Moqtada Sadr in this southern Iraqi city, leaving at least seven people dead and dozens wounded, according to officials at a local hospital.

Waving banners demanding the "expulsion of the outsiders," the crowd gathered near the Shrine of Ali -- a holy site for Shiites -- to call on the provincial governor to banish Sadr's Mahdi Army militia. Many residents of Najaf blame Sadr for heavy damage the city sustained during a Mahdi Army-backed Shiite uprising against U.S. forces a year ago.

Witnesses attributed Wednesday's violence to rifle-toting special forces from Iraq's Shiite-dominated Interior Ministry who had been deployed to the scene. Aides to Sadr said the final death toll could rise substantially.

"This is a savage action done by uncivilized people. They don't have any Iraqi honor," said Nassar Zeghayir Rubaie, a National Assembly member aligned with Sadr.

Iraq's health and transport ministers and 21 other lawmakers who belong to a political movement inspired by Sadr's deceased father, a revered Shiite cleric, said they would boycott their positions in protest. Mahdi Army forces were put on "emergency status" across the country, Rubaie said.

Iraqi security forces imposed an overnight curfew, and U.S. soldiers arrived in Najaf after midnight, blocking all entrances into the city. A civilian driving toward the American troops was shot and killed at about 1:30 a.m. Thursday.

In Baghdad, news services reported late Wednesday that Sadr's forces attacked three offices of the Badr Organization, a rival Shiite militia that often operates as part of Iraq's security forces, particularly in the south.

The fighting fueled fears of a violent rift among Iraqi Shiites, who are a majority in the country of 26 million people and hold the most powerful positions in the transitional government.

Sadr's Mahdi Army, largely composed of Shiites from impoverished sections of Baghdad and several southern cities, has long had tense relations with the Badr militia, which is the armed wing of the Supreme Council for the Islamic Revolution in Iraq, a powerful Shiite political party. The militias had not previously fought openly.

The clashes came as political factions across the country, including Sadr's supporters, continued to stake out positions on Iraq's constitutional referendum, slated for Oct. 15. The proposed constitution, submitted to the National Assembly on Monday with some disputed articles left open for discussion, will likely be ratified Thursday.

Most Shiites are expected to vote in favor of the constitution, the drafting of which was dominated by key figures of the Shiite-led government. Sadr, an outspoken nationalist and critic of the U.S. presence in Iraq, has not yet declared his position on the document. His supporters have threatened to join Sunni Arabs in voting against the constitution if it devolves power from Iraq's central government to autonomous regions.

"We are ready, by a single phone call within a minute, to defeat the constitution by voting against it in six provinces: Diwaniyah, Samawah, Nasiriyah, Amarah, Basra and Sadr City in Baghdad," said a Sadr spokesman, Jaleel Musawi.

Highlighting their potential alliance, hundreds of Sunnis and Sadr followers gathered Thursday in the northern city of Hawija to advocate opposition to the constitution.

"The Sunnis and their supporters among the Shiites will continue to demonstrate to safeguard the unity of Iraq, and reject partition and division," said Rakan Abdulla, a city council member from the nearby city of Kirkuk.

In Baghdad, as politicians gathered in an attempt to resolve remaining disagreements over the constitution, leaders of two of the country's most prominent Sunni organizations, the Muslim Scholars Association and the Sunni Bloc, criticized the drafting process.

"Our conference believes that if this draft is approved by the National Assembly as it is now, it will be illegitimate and doesn't represent all the Iraqi factions," the leader of the Sunni Bloc, Adnan Dulaimi, said at a news conference.

Hussein Shahristani, a Shiite who is deputy chairman of the National Assembly, said that during Wednesday's negotiations, Sunnis were hoping to eliminate a section barring former members of Saddam Hussein's Baath Party from some government posts and were seeking to empower the country's parliament to veto attempts by provinces to form regional governments.

Violence flared across Iraq on Wednesday, including in Baghdad, where three car bombs exploded but inflicted few casualties. News services reported that more than 40 insurgents carrying rifles and rocket-propelled grenades attacked a police station in western Baghdad, killing more than a dozen police officers and civilians.

Finer reported from Baghdad. Correspondent Anthony Shadid and special correspondents Omar Fekeiki and Bassam Sebti in Baghdad contributed to this report.

Shiites carry a banner reading: "No for politicizing the old city of Najaf." Clashes broke out in the city between Moqtada Sadr's supporters and other Shiites.