BAGHDAD, Iraq - Explosions and gunfire rocked the turbulent city of Fallujah for a second day today after coordinated attacks in other Iraqi cities killed about 100 people less than a week before Iraq's new government takes power.

U.S. tanks and armored vehicles maneuvered on the highway near the edges of Fallujah, firing in all directions, while armed men in an eastern suburb returned fire, witnesses said. Seven people died in two days of exchanges there, hospital officials said.

Hours later, a roadside bomb exploded in a residential neighborhood in Baghdad, killing one Iraqi policeman and wounding another, police said.

The attacks on security forces fit a pattern of violence that targeted several cities Thursday, when insurgents set off car bombs and seized police stations in an offensive aimed at creating chaos just days before the handover of power to a new Iraqi government. U.S. and Iraqi forces regained control in heavy fighting, but the day's violence killed about 100 people, most of them Iraqi civilians.

Abu Musab al-Zarqawi's terror network claimed responsibility for Thursday's attacks. Three U.S. soldiers were among the dead. At least 320 people were wounded, including 12 Americans.

As the situation worsened, Iraq's interim vice president warned that a deterioration in the country's security could result in emergency measures or martial law - however undesirable that may be in a democratic society.

"Announcing emergency laws or martial law depends on the nature of the situation. In normal situations, there is clearly no need for that (step)," Ibrahim al-Jaafari, a Shiite and member of the Islamic Dawa Party, told The Associated Press in an interview.

"But in cases of excess challenges, emergency laws have their place," he said, adding that any such laws would fall within a "democratic framework that respects the rights of Iraqis."

Mosul residents said the northern city was tense today, with a marked increase in the number of police on the streets. Fewer people ventured out to markets for fear of more attacks.

Elsewhere, three mortar shells exploded early Friday near an oil pipeline damaged last week by sabotage, police Capt. Mushtaq Talib said. The latest explosion caused no damage, Talib said.

Iraq's new leaders recently have suggested the possibility of at least partial martial law in some hotspots around the country as a way of stemming the violence. It is unclear, however, whether U.S. officials would go along with the idea.

The U.N. Security Council resolution approved this month gives the United States a primary security role in Iraq even after the transfer of sovereignty Wednesday.

U.S. and Iraqi forces regained control in heavy fighting and American forces set up checkpoints around Iraq to intercept weapons, guerrillas and bombs. They fear that al-Zarqawi, the militant who claimed responsibility for the offensive, plans a string of car bombings in Baghdad, said Col. Michael Formica, commander of the 1st Cavalry Division's 2nd Brigade.

"There is clearly a transnational threat, as represented by al-Zarqawi, and that threat appears - based on what we've seen in Fallujah and Mosul today - to want to bring the attack to Baghdad," Formica said Thursday.

A large number of victims from Thursday's attacks were killed in simultaneous car bombings in Mosul, but some also died as U.S. troops battled the guerrillas.

"We underestimated the nature of the insurgency that we might face during this period, and so the insurgency that we are looking at now ... has become a serious problem for us," Secretary of State Colin Powell told the British Broadcasting Corp.

The U.S. military responded with heavy firepower, dropping 11 500-pound bombs and a 2,000-pound bomb.

The assaults were launched Thursday morning, when black-clad guerrillas attacked police stations and government complexes in Baghdad, Baqouba, Mosul, Ramadi and Mahaweel. U.S. troops and insurgents traded heavy fire on the outskirts of Fallujah, where explosions were also heard early today.

The heaviest fighting was in Baqouba, northeast of the capital, where guerrillas shot their way into a government office complex, seized two police stations and destroyed the home of the provincial police chief. The stations were recaptured later, said Maj. Neal O'Brien of the 1st Infantry Division on Friday.

Two American soldiers died in the Baqouba fighting, the 1st Infantry Division said.

Insurgents also attacked a police station in a Baqouba suburb late Thursday, killing three officers and injuring one, said Dr. Nassir Jawad, who is in charge of the Baqouba morgue. Isolated skirmishes were also reported into Thursday evening, O'Brien said Friday.

But the day's worst bloodshed came in Mosul - the country's northern metropolis often touted as a success story in restoring order in Iraq - where the U.S. military said 62 people were killed, including a U.S. soldier, and more than 220 people were wounded.

Most died when at least four car bombs rocked the police academy.