Armed insurgents rampaged Thursday through Mosul, Iraq's third-largest city, detonated a massive car bomb in the capital and apparently seized control of two smaller urban centers. This violence took place as U.S. forces continued their major offensive in Fallujah.

The scattered and spreading guerrilla attacks appeared to be part of a threatened effort by insurgents to open new battle fronts away from Fallujah, an anti-American bastion 35 miles west of Baghdad in the Sunni Triangle.

Masked men brandishing machine guns and wrapped in ammunition bandoliers overran police stations in Mosul, a major city 220 miles north of the capital, carrying off weapons and armored vests in a second day of street violence, U.S. military officials here said.

In Baghdad, gunfire and explosions continued to rattle sections of the city, while gunmen battled U.S. Army units and Iraqi police in western neighborhoods largely populated by Sunni Muslims and officials of former president Saddam Hussein's Sunni-led government.

In a late-morning attack on Sadoun Street, a busy commercial strip, a car bomb exploded with a force that stunned even jaded residents.

The blast killed 17 people, blackened one block of the street, destroyed a medical supply store and incinerated 10 cars. The suicide bomber had apparently tried to hit a passing convoy of five Iraqi police cars.

"These are the Arab fighters who are losing now in Fallujah. I saw a whole family burned in front of me," said Abu Adullah, as a tear rolled down his cheek. Adullah's real estate office was damaged in the blast.

"May God curse them," he said. "May God curse them."

Thae Khudhair Jasim, 23, suffered wounds to his neck and chest when the windshield of his taxi shattered, showering glass over him and his passengers, a woman and her three daughters. All five survived.

"No one should see what I saw: pieces of flesh, cut legs, burned bodies," Jasim said.

Clashes in the capital erupted near the headquarters of the Association of Muslim Scholars and a mosque known for its militancy. The association, which represents Iraq's Sunni Muslim clergy and vocally supports the insurgency, said U.S. troops had raided the homes of two of its senior officials, including Harith Dhari, the secretary general.

American troops also participated in a raid on Baghdad's Ibn Taymiya mosque that resulted in the detention of Mehdi Sumaidi, a militant Sunni cleric. Sumaidi heads the strict Salafist movement in Iraq and recently threatened to issue "a general call to arms" over Fallujah.

The call appeared to have already gone out, judging by the number of attacks in the capital during the past 48 hours. On Wednesday, U.S. forces were assaulted 66 times by gunfire, mortars, rocket-propelled grenades, roadside bombs or car bombs.

In Mosul, U.S. troops mounted offensive operations early Thursday afternoon aimed at reclaiming the captured police stations and a food warehouse that was also being held. The warehouse was considered significant for its association with voter registration, which is in turn linked to Iraq's food rationing system. Insurgents have threatened Iraqis who take part in the elections, scheduled for late January.

Attack helicopters swooped overhead, and residents were warned to avoid the city's bridges, which insurgents were battling U.S. and Iraqi security forces to control. Five members of the Iraqi National Guard were killed in a battle on one bridge. American troops were responding to a call from the governor of Nineveh province, who imposed a curfew Wednesday.

"In several cases, anti-Iraqi forces exceeded the capabilities of the police on site, requiring reinforcements," said a statement issued by the U.S. military.

Insurgents have targeted Westerners and Iraqi security forces for months in Mosul, where the mostly Sunni population has long had a reputation for nationalism. In recent weeks, the city has also become known for intolerance, with vigilantes harassing and even killing members of its Christian minority.

Gunmen also roamed the streets of Baiji, an oil refining center in the Sunni heartland north of Baghdad, and Salman Pak, a small city in the band of angry towns that ring the capital's southern flank.

In Washington, several military experts said it was no surprise that bombings and insurgent attacks were occurring in other cities. Some said they saw Fallujah as symbolically significant but not the epicenter of the insurgency.

"It was a very important symbol, a dangerous ulcer," said Anthony H. Cordesman, a senior analyst with the Center for Strategic and International Studies. "This will be gone, and that is important. That doesn't mean there won't be problems in Baghdad, Mosul, Baqubah or Samarra, and that's easy to predict because it's already happening."

But Cordesman added that Fallujah's role as a recruiting station, haven and clearing point for foreign insurgents made it an important target.

On NBC's "Today" show Thursday morning, Air Force Gen. Richard B. Myers, chairman of the Joint Chiefs of Staff, said the nature of any insurgency was that people fight one minute and blend into the surroundings the next. He said the central goal of the offensive in Fallujah was not so much suppressing the insurgency as making it possible for people to vote in January.

"If anybody thinks that Fallujah is going to be the end of the insurgency in Iraq, that was never the objective, never our intention, and even never our hope," Myers said. "We're exactly on plan."

Staff writer Josh White in Washington contributed to this report.

A masked insurgent carries off a police flak jacket and rocket-propelled grenade launcher from one of several stations seized in Mosul, northern Iraq.