BY FRANCES ROBLES

  BOGOTA - Colombian president Alvaro Uribe's first day on the job began before dawn Thursday, when in the wake of explosive guerrilla attacks that killed 19 people, he inducted a group of citizen informants to guard hazardous highways.

  Uribe's spy squads were a cornerstone of his campaign, and show the kind of unconventional methods the new leader plans to execute to combat a 38-year leftist
  insurgency. Wednesday's strikes on Colombia's inauguration ceremony set the stage for escalated warfare -- and a presidency that is determined to end it.

  ''We have to stop being afraid,'' Uribe said. ``We're going to act with strength and determination to reestablish peace, and we call on all Colombians to cooperate with the armed forces with the aim of defeating the violent minority.''

  Uribe took office 3 p.m. Wednesday, just as homemade mortars flew through the southern Bogotá sky. One slammed into Casa Nariño, the presidential palace just a block from the capitol building where Uribe was sworn in before 600 dignitaries. Other explosives missed their targets and hit a shantytown, where 19 people -- among them three little girls and a number of vagrants -- died.

  UPHILL BATTLE

  The offensive was widely believed to be the first of a long and bloody campaign planned by the Revolutionary Armed Forces of Colombia's urban militias. It illustrated the uphill battle Uribe faces, and the ugly consequences of moving rural warfare to an urban landscape. Experts say Wednesday's assault will also serve to further validate Uribe, considered a right-winger willing to end the war at any cost.

  It was no coincidence that Uribe's first act as president was to kick off his blueprint for civilian resistance, a plan to mobilize one million volunteers in an effort to quell the violence that claims 3,500 lives a year. In places like Valledupar, informants will guard against illegal roadblocks at which people are frequently snatched from their cars and held for ransom. In addition to the first group installed in northern Colombia on Thursday, Uribe said he will enlist another 2,000 in Bogotá.

  ''If it hadn't been for what happened yesterday, these teams of informants would have looked fascist,'' said Pablo Franky, a political science professor at Javeriana
  University. "Yesterday's actions legitimize Uribe's authoritarian rhetoric, and allows the state to take extreme measures.''

  If there is concern among human rights groups about Uribe's proposal, that uneasiness hasn't trickled down to the Colombian populace. After vowing to offer a ''firm hand'' against FARC rebels, Uribe won an easy victory May 26. Wednesday's explosions only further enraged a war-weary public who understand that by electing the former Antioquia governor, the conflict will get worse before it gets better.

  'People are thinking, `Let's have two or three tough years and get it over with.' I've never seen that before,'' said Ambassador Luis Moreno, Colombia's emissary in
  Washington. "Alvaro Uribe is very centered, focused. He is convinced -- totally convinced -- he can have a huge impact.''

  MANY CHALLENGES

  Among the president's many challenges, Moreno said, are to establish a national security strategy that clearly defines victory. Uribe's predecessor, Andrés Pastrana, was widely criticized for initiating a peace process that lacked direction and government commitment.

  This president will have the benefit of $1.7 billion in U.S. aid, initially geared toward anti-narcotics and now ready for use to battle insurgents financed by the drug trade.

  Uribe has insisted that his other weapon in the war -- civilian operatives -- will be unarmed. They will not, Uribe said, take the route of the Convivirs, a similar national program outlawed when some participants allied themselves with illegal right-wing paramilitary armies.

  'People say, 'well, aren't these informants just going to get killed?' '' Uribe said. 'If we have one, two, or 10, sure they'd be killed. But if we have thousands or tens of thousands, then they would stand together and say, `They'll have to kill us all.' ''

  FEAR OF RETRIBUTION

  But doubt lingers about whether the plan would work in a country where people deeply fear retribution both from FARC rebels and their paramilitary enemies.

  Wednesday's explosive devices were launched from the patios of middle-class homes rented two months ago. Bogotá Mayor Antanas Mockus said several residents noticed something shady, notably that the houses' backyards were cloaked with tarp. Nobody called police.

  Metropolitan Police officials insist that the failure was not in a shoddy security operation or faulty intelligence, but in the lack of people willing to stick their necks out and report suspicious characters.

  ''There was carelessness,'' said Bogotá Police commander Gen. Héctor Darío Castro, ``and a lack of solidarity from the people.''