Thousands of U.S. and Iraqi troops pushed into the insurgent-held city of Fallujah just after sunset Monday in the largest military operation in Iraq since the U.S.-led invasion last year.

U.S. Marine and Army units entered Fallujah from the north, their armored vehicles crawling over huge dirt mounds that insurgents built around the city. Fighters could be seen digging positions as the U.S. forces moved forward.

Since a siege of the city of 300,000 people by U.S. Marines was called off in April, insurgents have controlled it. Located in the Sunni Triangle area west of Baghdad, Fallujah has been described by U.S. and Iraqi officials as a hub for the campaign of violence aimed at destabilizing Iraq's interim government and driving foreign military forces from the country.

In Washington, Defense Secretary Donald H. Rumsfeld vowed that the assault on Fallujah would not be called off this time. "I cannot imagine that it would stop without being completed," Rumsfeld said at a Pentagon news conference.

Army commanders said their troops encountered some resistance in the first hours of the battle, which began about 7 p.m. local time (11 a.m. EST). Soldiers and Marines left their armored vehicles to fight in the streets and clear buildings where insurgents were believed to be hiding.

Commanders said the American armored units will be followed by Iraqi forces, who will be largely responsible for clearing and securing parts of the city. The heaviest fighting, they said, is expected in the Jolan neighborhood and in industrial areas.

The Iraqis will also be responsible for engaging insurgents around mosques and other sensitive sites, the military said. Witnesses said U.S. fighter jets bombed three mosques in Fallujah on Monday. Military commanders said insurgents had been seen moving weapons into the mosques in recent days.

No information on casualties was immediately available Monday night. Radio transmissions indicated few American troops were wounded in the first five hours of fighting; Iraqi casualties could not be determined.

Residents reported that a Saudi national known as Abu Waleed Saudi, a senior military aide to insurgent leader Abu Musab Zarqawi's, was killed in fighting west of Fallujah.

As the battle escalated in the city, several explosions echoed through Baghdad, 35 miles to the east. Two car bombs detonated outside churches in the capital's southern section, news services reported. Several hours later, mortar shells hit outside the hospital where wounded survivors of the church bombings had been taken, apparently targeting Iraqi police and National Guardsmen maintaining order outside the emergency room. At least six people were reported killed in the attacks.

In separate violence, a U.S. soldier was killed by small-arms fire while on patrol in eastern Baghdad and a member of Britain's Black Watch battle group was killed when a roadside bomb tore the wheels off an armored vehicle south of the capital.

[On Tuesday, as U.S. forces pushed deeper into Fallujah after airstrikes overnight, the Reuters news service said one of its reporters saw a U.S. helicopter shot down over the city. It also cited police as saying that insurgents had attacked three police stations in the town of Baqubah northeast of Baghdad, killing 25 policemen and wounding many more.]

A few hours before the launch of the assault on Fallujah, called Operation Phantom Fury, interim Prime Minister Ayad Allawi visited Iraqi troops at their training base outside the city.

"The people of Fallujah have been taken hostage . . . and you need to free them from their grip," Allawi told the soldiers. "Your job is to arrest the killers, but if you kill them, then so be it."

"May they go to hell," the soldiers shouted. "To hell will they go," Allawi replied.

Allawi imposed a 24-hour curfew on Fallujah and nearby Ramadi starting at 6 p.m. and banned weapons in the two cities. He also closed border crossings with Syria and Jordan and shuttered Baghdad's international airport for 48 hours.

The border closing, which does not apply to overland food shipments, was intended to prevent foreign guerrillas from moving into Iraq and joining the insurgency. U.S. and Iraqi officials have described the insurgent force in Fallujah as a mixture of Iraqi and foreign guerrillas numbering as many as 3,000.

The Association of Muslim Scholars, which represents Iraq's 3,000 Sunni Muslim clerics and supports the insurgency, warned Iraqi soldiers against taking part in the offensive. In a statement, the group urged Iraqis against "being deceived that you are fighting terrorists from outside the country, because by God you are fighting the townspeople and targeting its men, women and children, and history will record every drop of blood you spill in oppressing the people of your nation."

Recent visitors to Fallujah have said it is largely empty of women, children and the elderly, but that a large majority of the military-age men who remain are "sons of Fallujah." Allawi brushed aside the notion that most of the insurgents in the city are Iraqis.

"I think there is a misperception," Allawi told reporters. "There is a division between the Iraqi people and the terrorists. We are after the terrorists. We are not after anybody else."

Recalling the massacre of 49 Iraqi National Guard recruits over the Oct. 23-24 weekend by a group led by Zarqawi, Allawi declared: "They say they are targeting multinational forces. They are killing and massacring the Iraqi people!"

"They think that Iraq now is weak, but I warn them from this place that the time of action has begun, and I will never allow anyone to inflict harm on the Iraqi people, whether they are foreign terrorists or the mercenaries of Saddam Hussein."

The prime minister said emergency powers that his government invoked on Sunday would be used selectively to target other cities where insurgents hold sway. "We are going to confront all the hotbeds in Iraq and get rid of all the terrorists," he said.

In interviews Monday, several Iraqis said they welcomed the government's hard line, calling the imposition of emergency powers long overdue.

"This law should be implemented in a tough, firm way, because there is a certain softness in this government," said Wadhah Jarrah, 60, a lawyer. "Why aren't those who kill police and National Guardsmen in cold blood tried and hanged?"

Sundis Abdallah, 52, a government employee, said: "This law is our last hope to achieve security. . . . These are the forces that are supposed to protect us. If they are killed, then who will protect the people, the state, the children? Criminals should be dealt with firmly so that the rest of the state can go forward."

But at his cigarette stand in Baghdad's Karrada district, Munaf Jabbar wondered what all the fuss was about.

"We have been living in emergency since this government was formed," he said. "Let them not deceive themselves. Imposing a curfew? It is already imposed. If you go out at 8 or 9 at night, you won't find anybody in the street."

In the hours before troops advanced on Fallujah, U.S. warplanes and gunships dropped bombs and fired artillery rounds into the city, targeting insurgents' defensive positions and blowing up booby traps intended to stall the offensive. The air assault lit the city in a giant explosion of orange, followed by the white twinkle of falling embers.

"Basically there's a lot of stuff blowing up right now," said Lt. Todd Hildebrant, 28, of Grafton, Mass., who was one of several soldiers from Task Force 2-2 of the Army's 1st Infantry Division watching the fireworks from their operations center outside the city.

Military commanders said the advance from the north was intended to surprise the insurgents, who they believe were expecting an assault from the west, a relatively quiet part of the city that U.S. forces easily took on Sunday night.

U.S. and Iraqi forces stormed Fallujah General Hospital late Sunday night without firing a shot, said 1st Lt. Lyle Gilbert, a spokesman for the 1st Marine Expeditionary Force. But when insurgents fired rockets at the building, and when the Marines securing nearby bridges tried to push forward slightly, insurgents launched a counteroffensive that resulted in a five-hour gun battle, witnesses said.

Col. John R. Ballard, commander of the Marine 4th Civil Affairs Group, based in Washington, said the military had been planning for weeks to secure the hospital as a prelude to a potential battle.

"We've surrounded it to protect it," Ballard said. "The key word here is to protect."

Rafe Hyad, the hospital's general manager, said U.S. forces locked him in a room after breaking down the doors.

They "ordered me not to go out," he said. "They searched all the rooms, asking for the reason why every one of the patients is here. We have a lot of pregnant women and premature children in the hospital."

Witnesses said U.S. airstrikes hit another hospital that had been established about three months ago by the former president of the United Arab Emirates.

Vick reported from Baghdad. Special correspondents Omar Fekeiki near Fallujah and Khalid Jaffar and Bassam Sebti in Baghdad contributed to this report.

Insurgents wielding small arms and mortars attack U.S. forces that fought their way into Fallujah after seizing a hospital and two bridges.Iraq's interim prime minister, Ayad Allawi, urges Iraqi troops to free the people of Fallujah "taken hostage" by insurgents.