BAGHDAD, Iraq - Insurgents loyal to radical Shiite cleric Muqtada al-Sadr battled fiercely with U.S. and Iraqi forces in the holy city of Najaf today and fighting quickly spread to other Shiite areas, threatening a shaky two-month-old truce.

"The cease-fire is over because of the actions of the occupation forces, and the situation has started to deteriorate," warned Sheik Abdul Hadi al-Daraji, a spokesman for al-Sadr in Baghdad.

Al-Sadr's men also fought with U.S. troops in the Baghdad neighborhood of Sadr City, shot at government offices in the southern city of Amarah and clashed with British forces farther south in Basra.

Combat in Najaf killed at least two people, and the U.S. military said one of its UH-1 helicopters was hit by gunfire and crashed, injuring the crew.

Also today, a suicide car bombing at a police station south of Baghdad killed five people and wounded 27, the Interior Ministry said.

The fighting was the worst flare-up between authorities and al-Sadr's forces since a series of truces two months ago ended weeks of violence that began after the U.S.-led occupation authority closed al-Sadr's newspaper and arrested a key aide. The newspaper was recently allowed to start printing again, but tensions had been rising in recent days between al-Sadr's Mahdi Army and Iraqi and U.S. forces.

People in Najaf said al-Sadr loyalists attacked a police station with mortars, rocket-propelled grenades and gunfire early today. U.S. Marines later entered the area, residents said.

The Marines intervened "to help the policemen protect the police stations and the city," Najaf Gov. Adnan al-Zurufi told the Al-Jazeera television station.

Al-Zurufi, who said two people were known killed, warned of "very bad consequences" if the militiamen did not disarm and leave the holy city.

Residents said busloads of Mahdi Army militants were seen entering the city.

The dome of the Imam Ali Shrine was slightly damaged in the fighting, witnesses said. Al-Sadr's followers announced over the mosque's loudspeakers that the dome was hit.

The intensely revered shrine, reputed to hold the remains of Imam Ali, the most hallowed saint in Shia Islam, was slightly damaged twice during fighting in May. U.S. forces have been careful to avoid damage to shrines in Najaf and other holy cities for fear of enraging Iraq's Shiite majority.

U.S. forces were about a half mile from the mosque, while al-Sadr's fighters were taking their injured into the mosque compound, witnesses said.

In Baghdad's Sadr City neighborhood, insurgents fired on a U.S. patrol, triggering a gunbattle, said Maj. Philip Smith, a U.S. military spokesman. There were no U.S. casualties.

Al-Daraji, al-Sadr's spokesman, said the fighting broke out in Baghdad because of the violence in Najaf and escalating tensions between U.S. forces and al-Sadr's loyalists throughout the country.

In Amarah, an appeal for Mahdi Army members to mobilize rang out from mosque loudspeakers and militants took to the streets, shooting at government buildings and blocking roads, residents reported.

Residents in the north of Basra reported a clash today between Mahdi Army fighters and British troops that wounded three militants.

The Mahdi Army earlier had said it was taking up positions close to where British troops normally patrol after a noon deadline passed in its demand for the release of four al-Sadr supporters detained two days earlier.

The British had not received a formal ultimatum, "only rhetoric," said a British spokesman, Maj. Ian Clooney. He said the men in custody had been detained for questioning.

An al-Sadr spokesman, Sheikh Assad al-Basri, said the militant group "prepared 1,000 fighters in Basra to confront the British forces who failed to respond to our demands."

Meanwhile, in Mahawil, 50 miles south of Baghdad, a pair of gunmen dressed in police uniforms opened fire on guards outside a police station while a third sped toward the station in a vehicle filled with explosives and blew up, killing five people and wounding 27, the Interior Ministry said.

The blast damaged the gate of the station and a dozen nearby cars and left a 15-foot-wide crater.

"I was outside the building when I saw a car heading toward us. We started shooting. I'm sure we shot him but he managed to explode the car," said police Capt. Adel Omran, whose leg was hit by shrapnel.

The two gunmen escaped, said Sabah Kadhim, an Interior Ministry spokesman.

"What do these criminals want from Iraqis? They sometimes target the Sunnis, the Shiites and the Christians and other times they target the police and the army. They, however, do nothing to the Americans," said Zayd Hadi, a civilian who was outside the station and suffered wounds to his face and stomach.

Near the town of Samarra north of Baghdad, the U.S. military said it staged a series of raids on suspected insurgents and detained nine people. Operation Cajun Mousetrap II began early this morning, targeting groups suspected of planning and financing attacks on Iraqis and coalition forces, the military said.