The attack continued into Thursday, leaving six Iraqis dead and 29 injured -- including eight children -- according to hospital officials.
"We sent a quick reaction unit to assist the policemen defending the station, but they were overwhelmed by al-Sadr fighters," said Najaf Gov. Adnan al-Zurufi. "We will solve this problem as soon as possible. We will ask for the help of the Americans, if necessary."
U.S. forces thus far have not engaged in the police station clashes.
The attack, which took place at the Ghari police station just yards from the revered Imam Ali Shrine, is the first wave of violence since a June 4 agreement between U.S. troops and militia forces ended weeks of violence in the region.
Al-Sadr had agreed to pull back from Islamic shrines in Najaf and Kufa, giving Iraqi police control of security. The U.S. Army also said they would not approach the holy sites, in order to give Iraqi security more control.
Newly named interim Iraqi Prime Minister Iyad Allawi responded to the attack by saying he would deal firmly with militia groups causing upheaval in Iraq.
"We have called upon him (al-Sadr) and others to abide by the rule of law and to respect peaceful means. Any continuity of using force will be dealt (with) by the Iraqi government in a very serious and strong way," he told reporters in Baghdad.
According to police, the attack started brewing when authorities attempted to arrest suspected thieves at a bus station next to the police headquarters Wednesday. Attackers opened fire and threw grenades near the headquarters. Police then returned fire, killing one of the gunmen, according to al-Sadr spokesman Qais al-Khalizi. The dead gunman's relatives then retaliated by attacking the headquarters, and fighting subsequently spread to the Ghari police station.
Also Thursday, gunmen claiming to be part of a militant Muslim group showed four men thought to be from a group of seven Turkish contractors kidnapped in Iraq. Dubai-based Al Arabiya television aired a videotape showing what it said was the gunmen standing behind the four Turkish men, who were on the ground holding passports.
Associated Press Television News also obtained the video, in which the gunmen said "our Jihad groups" had kidnapped the Turks because they worked for Americans, and called on Turkey to remove workers from Iraq.
"We urge the Muslim Turkish people to reject these acts and pressure Turkish companies to cancel contracts and pull their employees out of Iraq until we liberate our country and expel the occupiers from our land," the man said.
The gunman also said he "appreciated the stand of the Turkish people" against Iraqi prisoner abuse at Baghdad's Abu Ghraib prison.
A Turkish Foreign Ministry official told Reuters that it "appears seven people have been taken hostage. We don't have any concrete information on their identities."
The official said the Turkish ambassador in Baghdad and Turkish intelligence services were trying to secure the men's release.