After a day advancing on the Iraqi capital inch by inch, Marines hunkered down tonight for a nerve-wracking pause in a downtrodden neighborhood one Marine described as "hell on earth."

They had moved toward the center of Baghdad from the southeast, but stopped along a canal just short of the city limits because a bridge they hoped to use as an entrance had been sabotaged.

More than 100 Marines from the 1st Battalion, 7th Marine Regiment, took refuge on a few squalid blocks, where earlier in the day they had come under fire from rocket-propelled grenades and artillery. They were tired and nervous about possible attacks from the ramshackle buildings all around them, an urban scene of decay, shadows and uncertainty.

"We'll be in a very aggressive defensive posture tonight, sending out regular patrols, and establishing a presence," said Maj. Dan Healey, the commander of Baker Company, as his men scurried around an intersection, stepping over the rotting carcass of a donkey that was lying in the road. "It's tough, but it's better to step off tired tomorrow morning than not to step off at all."

The neighborhood where his Marines found themselves fit every Hollywood stereotype of a war zone, complete with bullet-riddled buildings, streets covered in garbage, and the booms of impacting artillery shaking the ground every few minutes. The population seemed less friendly than the Iraqi civilians the Marines had encountered in recent days. The people lining the streets did not wave or smile.

The Marines were greeted by the pungent stench of open sewers as their slow-moving convoy of armored vehicles rolled into town in the middle of the afternoon. The dilapidated duplexes offered assailants a dangerous vantage point from which to attack, so Marines peering from the hatches of their Amtrac amphibious assault vehicles scanned the rooftops of buildings for any signs of people toting weapons.

The buildings were so close to the street that a Marine leaning out of the top hatch could almost touch the walls. Sheets of cardboard had been placed in front of many of the windows, making it difficult to see inside.

They rumbled past a wall-sized mural of Iraqi President Saddam Hussein that had patches ripped from where his eyes and mouth would have been.

Warnings came in over the radio that earlier in the day Marines had been attacked by Iraqis firing from speeding ambulances and by civilians who had initially waved white flags to indicate that they were not hostile.

"When people come into your pause and don't stop on your warning, you waste them," ordered Lt. Aaron Lloyd, commander of the Amtrac platoon that was transporting the Marines.

When the Marines came to a stop, they began sending out patrols into the city's narrow streets. Cpl. Ryan Birrell led one group of eight Marines to explore the alleys running off of a main thoroughfare, four of them on each side of the street. They were trailed by two M1 Abrams tanks, one of which had a toothy plow on the front and the words "war pig" painted on its large cannon.

"There are friendlies on the right!" screamed Birrell, as he peered down a side street and saw Marines performing a similar patrol one block away. "And watch the civilians on that rooftop, we need to keep eyes on them."

The Marines passed a white-brick mosque, the minaret pockmarked with bullet holes, and ushered four men sitting on its stoop back inside the building by pointing with their rifle barrels. "Stay there," Birrell told them. "Don't come outside."

They continued past a small rusted Ferris wheel that was falling apart on an island in the road. The call to prayer echoed off the buildings, and a light rain began falling as daylight began to wane.

"What are we going to do when it gets dark?" Birrell asked his platoon commander, Lt. Seneca Todd, when they met at an intersection.

"Run some patrols, maintain security, as always," Todd replied.

The patrol returned to its starting point after an uneventful half hour, while another squad searching a section of town a few blocks away discovered a massive weapons cache, including more than 30 rocket-propelled grenades and several 80mm and 120mm mortar shells, in a building. They had searched the building when they discovered a large hole in one of the walls.

The stash was booby-trapped, they recalled, so the Marines loaded the building with C-4 explosives and leveled it with two loud blasts.

When night fell, Maj. Healey sent Marines to the rooftops of nearby buildings to monitor the streets with their night-vision goggles. Tank crews also took turns on watch with their thermal scopes. Marines were put on "50 percent security," meaning that half of them were required to stay on watch at all times.

It was not a restful night. Mortarmen slept in shifts within arm's reach of their mortar tubes. A group of mangy dogs kept many Marines awake, fighting and barking in the streets.

A primary concern for forces here was providing security for their Amtracs, which are lightly armored and vulnerable to any rounds larger than those fired by an AK-47 assault rifle. Earlier in the day, some of the Amtrac crews reported seeing an Amtrac from another battalion being towed back to the rear, its turret and front end heavily damaged. They speculated that it was probably hit by either artillery or a rocket-propelled grenade, but no further information was available.

Staying downtown for the night is "probably an [Amtrac crew's] worst nightmare," said Lt. Lloyd. "It's cloudy, so it will be extremely dark tonight, so even with our optics we are almost on a par with the enemy. It's definitely not ideal. I can't even imagine what Baghdad will be like."

A U.S. Marine with the 3rd Battalion, 4th Regiment, tends to a comrade in Baghdad after an artillery shell fired by Iraqi fighters struck his Amtrac amphibious assault vehicle.