A jumble of brick hovels pitched on a hillside overlooking downtown Medellin, Comuna 13 has long served as an urban redoubt for Colombia's two Marxist guerrilla armies, whose members have found money, sanctuary and sympathy among dislocated peasants driven here by war a generation ago.

There are no roads, only steeply sloping concrete stairs, dirt terraces and narrow pathways winding through shacks set into the hillside or perched on stilts. No map exists to guide Colombian soldiers through the maze. Nor do any of the roughly 160,000 people who live here, cowed by guerrilla threats or ideologically opposed to the government, often volunteer to do so.

It is a confusing battlefield. But Comuna 13 has become the most intense point of conflict in Colombia since Oct. 16, when President Alvaro Uribe ordered 3,000 troops backed by helicopters to move into the area. In its size and sweep, the operation has emerged as a significant demonstration of the new president's willingness to set aside fears of civilian casualties and criticism from human rights organizations to prosecute the more aggressive war he promised Colombians before taking office less than three months ago.

So far, Operation Orion, as the assault has been named, has resulted in 18 dead, 34 wounded and almost 250 arrests among the population of Comuna 13. But most of the people interviewed in Comuna 13 professed gratitude, saying they have been worn out by the guerrillas' long, demanding presence. In addition, more than 20 kidnap victims hidden amid the chaos of Comuna 13 have been set free, including the 13-member Castan~eda Higuita family, which had been held for ransom in its own home for weeks under threat of death by guerrilla captors.

The fighting has calmed now, giving way to scenes of army troops guiding hooded informants along the pathways in search of hidden guerrillas and their sympathizers. Children play soccer on dusty terraces. Old women and men sit on door stoops under the gathering rain clouds that sweep eastward each afternoon over Medellin, Colombia's second city. Most apparent, though, is what is missing: 18-to 30-year-old men, the rank-and-file of the guerrilla militias. They have vanished from the neighborhood as effectively as they once ruled it.

"At this point, we don't know where they are -- gone, hiding, arrested or buried," said Javier Machado, an assembly line worker in a plastic pipe factory whose home in Comuna 13 was riddled with bullets during the first day of the operation. "But if the army leaves us, they will be back."

The guerrilla response has been felt elsewhere. A half-dozen explosives, including a car bomb consisting of 80 pounds of dynamite that peeled the facade off of a downtown building, have been detonated in middle- and upper-class Medellin neighborhoods since the offensive began. No deaths have been reported here. But a bomb that exploded in the capital, Bogota, on Tuesday in front of metropolitan police headquarters killed two people and injured 36.

Although Colombia's 38-year-old civil war has been mostly rural, it has had various urban fronts, especially in this city of 2 million people situated 150 miles northwest of Bogota. Medellin, the capital of Antioquia province, has long been a hub of the enormous drug trade that helps finance the two largest warring irregular armies. It also is a guerrilla logistics and recruiting center and a gateway to the strategic northwest.

But the military has never undertaken something as large as Operation Orion in a city, signaling a new willingness by the U.S.-backed armed forces to engage guerrillas in important urban arenas as Uribe simultaneously moves to build up government security presence in the vast, lawless countryside.

For the Revolutionary Armed Forces of Colombia (FARC) and the smaller National Liberation Army (ELN), two insurgencies that have been trying to topple the government since 1964, the military's new tack poses a challenge. It threatens rebel logistics, financing and intelligence operations, which are rooted in important provincial capitals such as this one.

The FARC's 18,000 armed members and the ELN's 3,000 to 5,000 fighters rely on such support from urban networks, which frequently mastermind kidnappings for ransom, arrange arms shipments and collect profits from a drug trade that is the primary target of a $1.3 billion U.S. aid package to Colombia.

Emerging from a series of community self-defense groups formed to fend off drug gangs in the early 1990s, the guerrilla militias here have grown in size and influence. Without the counterweight of a government presence, Marxist indoctrination imparted by rebel political organizers took deep root in the desperate shantytowns.

In recent years, however, a privately funded paramilitary army, the United Self-Defense Forces of Colombia (AUC), has rolled back guerrilla militias here and in many Colombian cities. Although the AUC is outlawed, it often works alongside government security forces in an undeclared alliance.

Although Comuna 13 was a guerrilla stronghold, it was the exception in Medellin. Colombia's domestic security service estimates 70 percent of Medellin's comunas are controlled by the paramilitary forces and that most of its 400 armed gangs, comprising 10,000 members, are affiliated with the AUC.

Human rights advocates here have questioned why Colombia's military leadership chose to attack Comuna 13 rather than the more numerous AUC-controlled areas, a choice that Gen. Mario Montoya of the Colombian army, commander of the operation, has said was determined by guerrilla brutality in the area. Those tactics, he said, include the recent beheading of two bus drivers suspected of being government informants.

Medellin has long served as an unofficial laboratory for security measures that have relied on paramilitary groups to control guerrilla militias. The military-paramilitary model was first tried out here years ago. It gained currency during Uribe's tenure as governor of Antioquia province, from 1995 to 1997.

The first wave of army and National Police troops began pushing through the pre-dawn darkness on Oct. 16, moving down the hillside from the Tower, an electrical pylon used by guerrilla sharpshooters, toward the neighborhoods of Las Independencias, El Salado and 20 de Julio. U.S.-made AH-60 Blackhawk helicopters buzzed overhead, providing covering fire in what quickly became house-to-house combat.

The violence that has ensued in this neighborhood and others like it has exposed a statistical myth that has made Colombia's war appear less deadly than it actually is. While international human rights agencies and Colombia's Defense Ministry say 3,500 people died last year as a direct result of war, the figure does not include lives lost from the lawlessness that the conflict foments. In the last decade, 42,393 people have died violently in Medellin, many of them comuna residents who have not been counted as official victims of war.

Despite the treacherous urban terrain, the number of civilian deaths in the current army offensive has been limited to four of the 18 people killed in combat so far. Of the 14 non-civilians, Colombian authorities say 10 were guerrilla militia members and four were government troops. The statistics suggest that the army, while proceeding aggressively, has moved with caution under difficult circumstances.

Pathways curling through the neighborhood are sometimes barely wide enough for two people to pass. Many of the ramshackle houses share common walls. Guerrilla fighters knocked holes into many of these walls, making tunnels that allowed them to pass from one side of the comuna to the other without ever being outside.

Caterine Alejandra Franco, 13, watched the opening fight unfold from Colegio Carmelita Arcila, where she attends sixth grade, and lingered long enough to watch troops begin bringing the dead and injured out of her neighborhood. Most of the wounded were taken to an emergency room below El Salado, where a statue of a painted Virgin Mary -- ubiquitous in this most Catholic province -- stands above the words "pray for us."

Since the end of the initial fight, troops have been escorting captured militia members, faces hidden by balaclavas or camouflage bandannas, through the neighborhood. A pointed finger from one of them means a trip to the headquarters of the investigative branch of the National Police, where scores of suspects are being held in an open-air patio.

Freddy, 16, who lives in Las Independencias, said he was held from 11 a.m. Friday until 3 a.m. Saturday after police found a balaclava and a videotape in his house. He said he did not know what was on the tape, and that the hood had been tossed into his house by a guerrilla militia member fleeing police. He said he was treated roughly, an allegation corroborated by witnesses to his arrest, but not as badly as others he saw.

"They asked me, 'Where are the weapons? Where are the militias,' " said Freddy, who declined to give his last name for fear of retribution by government security forces. "I didn't say a word."

The tactics, while criticized by many residents, have also been hailed with equal enthusiasm by many comuna residents who are pleased to see at least a temporary end to the guerrilla regime. "We are so tired," said Machado, whose son Javier, 14, is of the right age to be pressed into obligatory service by the militias.

Comuna 13, a poor, sprawling neighborhood on the western edge of Medellin, has been the scene of the Colombian military's largest urban offensive against guerrilla supporters, with 3,000 troops backed by helicopters.