Swept aside by U.S. troops who drove through the streets of Baghdad, President Saddam Hussein's government collapsed today, ending three decades of ruthless Baath Party rule that sought to make Iraq the champion of a modern Arab world but left a legacy of fear, poverty and bitterness.

As U.S. Army troops occupied the west bank of the Tigris River and U.S. Marines rolled into the eastern part of the city, facing only scattered resistance, thousands of Baghdad residents poured into the streets to celebrate the government's defeat and welcome the U.S. forces in scenes of thanks and jubilation.

With pent-up fury, the crowds also rampaged through offices of the government and state-owned companies, lugging away everything from plastic chairs to Toyota pickups once doled out as patronage. In festive moments, others tested their newfound freedoms, engaging in noisy debates in the street and denouncing Hussein in words that would have brought a death sentence only days ago.

The feared Baath Party apparatus disappeared from the streets. Its junior officials and militia fighters, once posted at every intersection, were nowhere to be seen. Many were said to have changed into civilian clothes to escape detection. Party uniforms and weapons were scattered at sandbagged positions that only days ago had been vaunted as the heart of a bloody last stand. Along some streets, military vehicles stood bleak and deserted, testimony that a once-efficient administration had come to a halt.

The fall of Baghdad -- and its celebration by thousands of Iraqis eager to heap scorn on their leader -- marked a climactic moment and a clear turning point in the war launched by the Bush administration 21 days ago to take down Hussein's government and rid Iraq of what U.S. officials said was a store of weapons of mass destruction.

Since launching the invasion from Kuwait March 20, U.S. and British forces have seized control of all the country's important centers south of Baghdad and at least two-thirds of its territory. The Euphrates River city of Hilla came under U.S. Army control today, completing occupation of the Euphrates Valley. The seizure of Baghdad added to the list the seat of Hussein's government and the heart of Iraq's old and storied civilization.

But Hussein, 65, his family, his ministers and members of the ruling Revolutionary Command Council remained unaccounted for, having vanished from public view over the last several days as U.S. forces closed in. U.S. officials cited radio traffic from the remote town of Qaim, in the far west near the Syrian border, as an indication some Hussein followers might be hiding there. In addition, several major Iraqi cities have not yet been occupied by U.S. forces, including Tikrit, Hussein's home town, and Kirkuk and Mosul in the northern oil fields.

'This Is Not Over'

"There's a lot more fighting that's going to be done," Defense Secretary Donald H. Rumsfeld warned in Washington. "There are more people [who] are going to be killed; let there be no doubt. This is not over, despite all the celebrations on the street."

The Pentagon has identified 91 U.S. military personnel as killed in action or missing in action and about 400 wounded since the war began, a count that has often fallen behind reports from the field as information travels through the bureaucracy and families are notified. The number of Iraqi casualties has not been reliably compiled, but U.S. officials have estimated them in the thousands.

Although Hussein and his sons were targeted in an airstrike two days ago on the strength of intelligence that they were gathered at a meeting and vulnerable to attack, Rumsfeld said he did not know whether they were still alive.

"It is hard to find a single person," Rumsfeld said. "It is hard to find them when they're alive and mobile, it's hard to find them when they're not well, and it's hard to find them if they're buried under rubble. We don't know. And he's not been around. He's not active. Therefore, he's either dead or he's incapacitated, or he's healthy and cowering in some tunnel someplace, trying to avoid being caught. What else can one say?"

But in Baghdad, today was for merrymaking as, in quick succession, U.S. forces took control of the streets and the symbols of Hussein's rule fell. In Firdaus Square, dancing crowds aided by U.S. Marines toppled a 20-foot statue of the longtime leader, his arm raised in Stalinist fashion. With ropes, residents dragged its severed head through the streets, cheering along the way. Hussein's ubiquitous portraits, as early as this morning still gracing newspapers, were smashed. On one defaced picture, a devil's horns were scribbled in black.

"I can tell you the fear has lifted from people's hearts," said Faleh Hassan, sitting at Abu Ahmed restaurant in central Baghdad.

It was a startling collapse for a government that, only three weeks ago, had predicted Baghdad would become a quagmire for invading forces and declared, with bluster and bravado, that it was debating whether to bury U.S. and British troops in mass pits or individual graves. It followed one of the quietest nights of the war in Baghdad, with only sporadic shelling and the crackle of gunfire.

Government No-Shows

The fate of Hussein and his government was a mystery that intrigued Baghdadis as well as officials in Washington. But in Baghdad, there was no one to ask about it. For the first time since the war began, Information Minister Mohammed Saeed Sahhaf, whose comments had grown increasingly bizarre as the war unfolded, failed to arrive at the Palestine Hotel to deliver his daily briefing. Only a day earlier, he insisted -- with not a hint of irony -- that Baghdad was bracing "to pummel the invaders."

Other Iraqi officials, who appeared less and less in public as the war progressed, were also nowhere to be seen, including Hussein's sons, Uday and Qusay. Despite the bombing on Monday of a compound in the well-to-do Mansour neighborhood where Hussein was believed to be hiding, many residents of Baghdad expressed belief he survived and possibly went to Tikrit, the home base of his clan and many of his closest lieutenants.

Many spoke of settling scores with those officials. "If I see my enemy before my eyes, I will kill him," said Hassan, the restaurant owner. "To be honest, that goes for all Tikritis."

But scenes of celebration were more common. In images broadcast around the world, hundreds of Iraqis poured into Firdaus Square, where they headed for a statue of Hussein perched on a 20-foot pedestal of purple granite. First came a sledgehammer. Men took turns knocking chunks off the base, to the wild applause of the crowd. Then a rope, tied like a noose, went over the statue's head. Finally, Marines brought an M88 tank recovery vehicle. They tethered one leg, then two, before finally settling on a thick chain that went around the statue's neck. It fell halfway, then crashed to the ground.

All that was left was the twisted metal of his feet, two rusted pipes jutting out.

With the rage of grievances accumulated over a lifetime, members of the crowd beat the fallen statue with sledgehammers, rocks, chains and their feet. Some slapped their shoes on it. Others made off with its head, dragging it through the streets.

"It was a strong statue," said Stefan Abu George as he watched the scene unfold. "It's not strong anymore."

Conflicting Emotions

Down the street, crowds greeted U.S. troops with flowers, candy and, occasionally, kisses.

"We love you!" some shouted. Others, with more anger, cried out, "No more Saddam Hussein!"

Some scrambled for packaged meals-ready-to-eat the Americans handed out, almost setting off a riot near the tanks. Others picked flowers from a nearby park and distributed them to soldiers and anyone resembling an American. A few simply stood and stared, as curious as they were jubilant. For the first time in a half-century, troops were rolling down Baghdad's streets with a foreign flag.

Those conflicting emotions gave rise to odd moments.

"I'm not happy," George said. But when a tank rolled by, he waved. Then he declared, "I love Saddam, he's courageous, he's a hero." The words set off a boisterous debate in the streets, an argument of the sort not heard publicly in a generation.

"Saddam is a dog, a son of a dog," shouted Majid Mohammed, 47, an electrical engineer.

But even Mohammed's family bore the scars of a system that relentlessly tried to link its fate to Iraq's, its leader's destiny to its own.

"I'm sad," said Mohammed's 12-year-old daughter, Sara, as they left the scene. "They stole our freedom."

Her words pained Mohammed. "Until now, I haven't been able to speak my feelings about him."

Even during the jubilation, Hussein still felt present in Baghdad after such a long period as absolute ruler. He became the effective head of government soon after the Baath Party took over in a coup in 1968 and formally assumed the presidency in 1979. His sudden absence opened a horizon that was at once unknown and uncharted. Iraqis spoke freely, fascinated by saying words only expressed in private, and even then, in whispers. But there was a nagging sense that words were still monitored, that statements could come to haunt them.

"Are you sure the regime is gone?" asked Mohammed Abdel-Amir, 34, a Shiite Muslim from Karbala.

Others had more mundane worries. They asked when electricity and telephones would return, after a week-long interruption. For others, it was the more fundamental issue of their relationship to U.S. forces that arrived today as liberators. Even in the celebratory scenes in the streets, some expressed hope that the U.S. presence would not become an occupation. Others were unsettled by the presence of a U.S. flag atop a column of tanks and other armor in a city whose name still resonates across the Arab world for its medieval glory.

"They got rid of the oppression, there's no question about that. But we want to know how it turns out -- are they here for our sake or for the sake of oil?" said Shidrak George, 38, watching the crowds pull down Hussein's statue.

Reflection and Uncertainty

In a country where virtually every family has a tale of suffering at the hands of the Arab world's most brutal government, the day prompted reflection -- over the fate of a rich country left poor and over a dictatorship that proved relentlessly durable. After three decades of powerlessness, many braced for the claims that the disenfranchised Shiite majority would make.

The uncertainty revolved as well around the enormous task of U.S. forces in crafting a new government. Many asked whether Iraqi dinars, emblazoned with a portrait of Hussein, could still be used. If not, when would they change? Others asked when the United States would return telephone service, cut off last week in a move that left the city isolated and secluded.

The war began in Baghdad with a barrage of missiles at a compound on the city's western outskirts where Hussein was thought to be hiding. What followed was an air assault that, at times, terrified the population. Casualties stayed relatively light when bombing targeted the symbols of Hussein's rule -- largely deserted presidential palaces, intelligence headquarters and government offices. They rose dramatically a week later when the attacks broadened to include telephone exchanges and transmission towers in crowded neighborhoods and, even more spectacularly, when U.S. forces arrived in the city on Saturday.

Kindi Hospital, which treats many of Baghdad's civilian wounded, this morning reported one of its busiest days.

Some said they would not forget the toll the war has caused among civilians. But remarkably, many seemed to look past it.

That future seemed to make some Iraqis more anxious. How long would the Americans stay, what did they plan to do, what government would they bring and what were their real intentions? The questions suggested that the United States -- its credibility suffering in much of the Arab world -- had a window of goodwill in a capital that seemed genuinely thankful for the removal of Hussein's government. But how long that window lasts may become a more pressing question in time.

"It's up to the Americans what this becomes," said Nazir Mustafa, 46, watching the tanks roll down the street. "Maybe it will be colonialism, maybe it will be liberation from the regime. The truth will soon become apparent."

Staff writer Vernon Loeb in Washington contributed to this report.

A young Iraqi flashes victory signs as he walks with U.S. Marines in Baghdad, where jubilant crowds swarmed into the streets. Members of Lima Company, part of the 7th Marine Regiment, walk past Martyrs Monument in Baghdad during their operation to secure the city center. Marines faced scattered resistance when they rolled into the eastern part of the capital.

Lance Cpl. Shawn Hicks of Arizona gets a kiss from an Iraqi man as crowds celebrate the arrival of American troops in central Baghdad.