Loud explosions mixed with intermittent bursts of machine gun and antiaircraft fire echoed through Baghdad on Saturday morning as allied warplanes rumbled overhead and the city's defenders prepared for a final battle.

Fires started by the government to conceal targets raged through the morning haze.

By 9 a.m., there seemed to be much less traffic on the streets than usual for the start of the Muslim business week, even during the war.

Before dawn Saturday, huge flashes illuminated the night sky toward the southeast, where Iraqi defenders faced U.S. Marines. A steady pounding of distant explosions, some strong enough to set off car alarms in the center of the city, could be heard through the night.

On Friday, the capital's defenders prepared to make their last stand -- digging ditches and stocking up on ammunition, even as thousands of frightened residents fled in bumper-to-bumper traffic.

The military's preparations could be seen on the last three miles of the road heading south of Baghdad, where thousands of army troops and militiamen dug more trenches and foxholes.

Determined but not frantic, dozens relaxed in the shade against a wall or under a tree next to their weapons, taking shelter from an exceptionally hot April day.

But by nightfall, Baghdad had the appearance of a city under siege -- empty streets, darkness from a power outage that began Thursday evening, and isolation from the outside world after allied bombings destroyed the city's telephone exchanges.

The gloom reverberated with explosions -- a nightly occurrence since the start of the allied bombing on March 20. The only sound between the booms was the hum of generators.

On Friday, tens of thousands of the residents fled the city, streaming toward the north and to the province of Diala to the northeast, in six-mile-long lines of traffic.

They packed buses, trucks, pickup cars, taxis, private cars -- even horse-drawn carts -- with blankets, foodstuffs, furniture, heaters, television sets, pillows, stoves, cooking pots, mattresses and pillows. And they waited in long lines at gas stations.

The U.S.-led forces amassing at the city's outskirts, having taken control of the international airport, searched tunnels under it. Marines fought Fedayeen paramilitaries to the south, while some Army forces advanced with little strong opposition.

After dawn Saturday, U.S. tanks and armored vehicles made their first forays into the Iraqi capital, making reconnaissance probes.

It was not clear whether they would try to attack Baghdad or whether they might apply a loose siege to the city, as they did in Basra, in the south.

"There are several options to get to where we are going," said U.S. Central Command spokesman Capt. Frank Thorp in Qatar. "Going into Baghdad is one option. But there are many other options."

They are faced, though, with a budding humanitarian crisis. Allied officials insisted that they did not target the power grid because they knew that a loss of electricity would also cut water supplies.

But that has happened. And the result could be human suffering that could force the allies to move into Baghdad sooner than they would otherwise -- risking the kind of bloody urban warfare they hoped to avoid.

Iraqi President Saddam Hussein, in an address read on his behalf Friday by Information Minister Mohammed Saeed al-Sahhaf, seemed to relish a cataclysmic battle for Baghdad.

"Throughout history, evil invaders have targeted capitals and believed battle would be in the capital. But when the capital is steadfast and the invaders are defeated and repelled, they retreat in defeat," he said.

Later, Saddam appeared on Iraqi television, urging his followers to strike the attackers "forcefully." Arab television networks also aired footage of a man they said was the Iraqi president, walking among excited crowds cheering, "With our blood and souls we redeem you Saddam."

Saddam would like to be remembered as an underdog that had the courage to fight a seemingly invincible enemy. He repeatedly has betrayed a tendency to see history as largely being made on the battlefield.

And so the forces approaching Baghdad may face a tough battle from thousands of well-concealed troops and militiamen armed with anything from Kalashnikovs and artillery to multiple rocket launchers and mortars.

At the last Iraqi checkpoint, 12 miles south of Baghdad's center, an officer in full combat gear stood Friday in the middle of the highway peering south through his binoculars. Around him, soldiers dug ditches, shifted ammunition from trucks to fighting positions and stood guard.

Even if the city's defenses collapse in the face of the Americans' far superior firepower and their complete domination in the air, thousands of militiamen and hardcore members of Saddam's Baath Party may carry on fighting -- street to street and alley to alley.

The prospect of such a battle has been on the mind of the city's 5 million residents all along. Some have reacted to that prospect with the "let them come" bravado many in Baghdad like to flaunt, while others have shown genuine fright.

The battle for Baghdad has already taken its toll on the city's civilian population. Fighting at the airport Thursday night and Friday has engulfed nearby residential areas, where stray bombs and rockets, residents say, have landed on homes.

"I don't know what happened to my family," said Jassim Abded Mahdi, recounting how shrapnel fell on his family as they were having breakfast Friday morning in the backyard of their house in the Fruit district near the airport. He was wounded on his side.

"My twin brother Mohammed is being operated on and I don't know whether I'll ever see him again," said Mahdi, a 27-year-old taxi driver, his eyes welling up.

Other casualties in Al-Yarmouk hospital's surgery and emergency wards came from districts also close to the airport, like Radwaniyah, and they mostly suffered shrapnel wounds.