Insurgents launched a coordinated offensive against police and U.S. occupation forces in six Iraqi cities and towns Thursday, exploding car bombs and assaulting police strongholds in a string of attacks that killed scores of Iraqi police officers and civilians, as well as three American soldiers.

The attacks, which began at dawn and raged through the morning, were the broadest and among the bloodiest in an insurgency that has intensified markedly in the weeks leading up to the transfer of limited authority to the Iraqi interim government, scheduled for Wednesday.

Reports by the Iraqi Health Ministry, which received tallies from hospitals around the country, and the U.S. military indicated that about 100 Iraqis were killed and about 320 injured.

The widespread attacks, in a country still trying to reorganize its ripped-up government and security services, generated a flood of confusing and contradictory reports in Baghdad. But through the fog emerged a clear impression that insurgent forces have the strength, organization and support to mount multiple attacks in the face of 138,000 U.S. military personnel, about 25,000 other foreign troops and, by Pentagon count, more than 200,000 members of Iraqi security forces trained under U.S. supervision.

The interim government's prime minister, Ayad Allawi, said the attacks were not as well planned as they appeared and had been carried out by disparate groups united only in their opposition to the occupation. These, he said, included die-hard supporters both of toppled president Saddam Hussein's secular government and of Ansar al-Islam, an Islamic extremist group that sprang up in northern Iraq in the years before Hussein was pushed from power in April 2003 by invading U.S. troops.

Ansar al-Islam has been connected to Abu Musab Zarqawi, a Jordanian linked by U.S. officials to Osama bin Laden's al Qaeda network. But Zarqawi's recent declarations have been issued in the name of another group, Monotheism and Jihad. Some of the militiamen who attacked Iraqi police posts Thursday said they belonged to Monotheism and Jihad, which also issued a statement asserting it was behind the day of violence.

"Your brothers in the Monotheism and Jihad Group launched a wide assault in several provinces in the country, which included strikes against the apostate police agents and spies and the Iraqi army alongside their American brothers," said the claim, posted on an Islamic Web site and relayed by the Reuters news agency.

Allawi, responding to questions about the attacks, asserted as he has on several recent occasions that his government was preparing to strike hard against the insurgents as soon as it takes over Wednesday. Despite his declarations, which Iraqi officials have explained as an attempt to buck up the nervous public, U.S. military officers have made it clear that they, not the Iraqis, will be in charge of security for the foreseeable future.

"I'm sure that the unity of the Iraqis will stop these criminal operations, and Iraqis will win," Allawi said in front of television cameras. "I want to tell the Iraqis that there is no need to be afraid, that these are criminal operations. . . . I declare that the security forces -- the police and the army -- will defeat these criminal actions of the infidels, the killers of the Iraqi people."

The most sustained attack came shortly after dawn in Baqubah, a farming hub about 35 miles northeast of Baghdad. Gunmen firing AK-47 assault rifles and rocket-propelled grenades took over the town's main police station and three other government buildings. Others attacked a patrol of U.S. soldiers, setting off a clash in which two of the Americans were killed and seven were wounded, the U.S. military announced.

Militiamen wearing black uniforms and black masks were seen later patrolling the city center; policemen were nowhere in sight, despite Allawi's attempt to portray his new government as able and willing to assume authority. As the provincial police chief moved about the city seeking to rally his men, the insurgents set fire to his home, the U.S. military said. The triumphant insurgents then erected black flags atop government buildings that they had occupied and proclaimed themselves to be followers of Zarqawi's Monotheism and Jihad Group.

But their air of triumph dissipated when U.S. warplanes dropped three 500-pound bombs on a pair of buildings where the insurgents had gathered near the city's soccer stadium, said a report from the 1st Infantry Division, which has responsibility for the area. Smoke rose over the city, and the insurgent gunmen vanished from downtown Baqubah.

The Health Ministry in Baghdad said it received reports from Baqubah hospitals that 13 Iraqis were killed and 15 wounded. Local correspondents for Arabic-language television channels put the toll at 20 killed and 25 wounded, including a number of Iraqi policemen.

By far the deadliest attack came in Mosul, 220 miles north of Baghdad. Local police said five car bombs hit several police stations and a police academy near a hospital in a succession of blasts that began shortly after dawn. The U.S. military said late Thursday that 62 people were killed, including an American soldier, and more than 220 others were wounded.

Local leaders imposed a 9 p.m.-6 a.m. curfew, and the city television station warned residents to stay indoors for the "general good," Reuters reported.

U.S. Marines, meanwhile, clashed for several hours with gunmen from Fallujah, the restive city about 35 miles west of Baghdad that is home to various Muslim militias and, U.S. officials charge, a band of foreign Arabs drawn to Iraq to fight American forces out of sympathy with bin Laden and Zarqawi.

Maj. Gen. Zibar Zobaie, a former Iraqi army officer who speaks for the Fallujah Brigade, said the fighting began when a convoy from the 1st Marine Expeditionary Force was attacked by gunmen two miles east of the city. The gunmen were not from Fallujah but launched their attack from the city, said Zobaie, whose brigade is supposed to ensure security in Fallujah under an agreement with Marine forces.

Marines responded to the attack with snipers, helicopter gunships, artillery and armored vehicles, knocking down houses and killing and wounding civilians, Zobaie complained. As a result, Fallujah residents fired back, he said. A Marine AH-1H Cobra helicopter gunship, apparently hit by gunfire, was forced to crash-land, but its crew walked away unhurt, a Marine spokesman said.

"These were not fighters," Zobaie said on al-Arabiya satellite television. "These were Fallujans. Fallujans, when they come under attack, they defend their city, their women and their children."

In a statement to the al-Jazeera satellite television channel, Fallujah militia leaders threatened to set Iraq's oil pipelines and wells ablaze if the Marines continued firing on Fallujah. By mid-afternoon, however, a truce was worked out under which the Marines would cease firing and the Fallujah Brigade would guarantee that Fallujah's gunmen and militias would also stop firing and stay in the city.

The Health Ministry initially said hospital reports indicated nine Iraqis were killed and 37 were wounded in Anbar province, which includes Fallujah. The ministry later reported 20 killed in the province's capital, Ramadi, but did not specify a casualty count in Fallujah.

Insurgents in Ramadi attacked a police station at dawn, seizing control and gunning down seven policemen and seven civilians before planting explosives and blowing up the station, police told reporters. A bomb also exploded at another police station in Ramadi, but no one was reported killed.

In Mahaweel, near Hilla about 60 miles south of Baghdad, insurgents fired mortars at a police checkpoint, killing one Iraqi officer.

A man wearing the uniform of a police lieutenant carried a briefcase toward a joint U.S.-Iraqi checkpoint in southern Baghdad's Dora neighborhood, then went up in a tremendous explosion that killed him and four Iraqi policemen, bystanders and police recounted. The passenger cabin of a small pickup truck was reduced to scrap by the blast, but its load of fresh green beans was only slightly charred.

"This was a most cowardly attack, coming this way to kill the National Guardsmen," said Lt. Walid Ali, a 26-year-old guardsman who was directing traffic at the scene.

No U.S. casualties were listed, but reporters saw several U.S. soldiers tending to a comrade lying in the street nearby shortly after the explosion.

At about the same time, militiamen wielding automatic rifles and rocket-propelled grenade launchers attacked four police stations around the capital. The U.S. military said all were driven back by return fire from the police with assistance from U.S. troops.

Correspondent Scott Wilson in Baqubah and special correspondent Khalid Saffar contributed to this report.

Iraqi police secure the scene of one of five car bombings in the northern city of Mosul, where at least 62 people died.