Not long ago, the prospect of standing before a group of powerful police officers would have been a nightmare for Alcibiades Fuentes, a longtime member of one of the illegal militias responsible for widespread violence and much of Colombia's drug trade.

But Friday, Fuentes strolled before a group of policemen and was greeted with smiles and handshakes. Last year, he put down his gun and enrolled in a government program to reenter society, becoming one of the first to make the transition from lawlessness to legitimacy, through a process that the government hopes will be repeated thousands of times.

"The program is giving us the opportunity to start again and make a new life," said Fuentes, a former member of the United Self-Defense Forces of Colombia paramilitary group who hopes to be granted asylum in Canada eventually. "Why would someone go back to an illegal life where they've suffered, knowing we are fine here and that we are getting the chance to study?"

But the demobilization program, based on a new law, has aroused intense criticism from human rights groups, U.S. politicians and others. The critics say the incentives in the law will allow some of the country's most dangerous criminals to escape justice through lightened penalties and legal loopholes that could protect them from extradition. They say the government of President Alvaro Uribe, while well-intentioned, is being manipulated by international terrorists.

"The law doesn't really make an effort to dismantle these groups," said Cesar Gaviria, who was Colombia's president from 1990 to 1994 and now heads an opposition political party. "They will still have their full economic power, their political power, and they will be pardoned for everything they did."

The Uribe government hopes the law, passed last month, will prompt as many as 20,000 paramilitary members to lay down their guns and reenter legitimate society after years of battling Marxist rebels, often in concert with the Colombian army, killing thousands of peasants, assassinating politicians and financing their operations through the drug trade. The right-wing paramilitary groups were formed in the 1980s by large landowners and drug cartels to protect their interests from leftist guerrillas.

The U.S. government considers Uribe one of its closest allies in Latin America, and it has provided about $3.5 billion in foreign aid since 2000 to fight Colombia's illegal drug industry, responsible for as much as 90 percent of the cocaine entering the United States, according to U.S. government figures. U.S. officials expect to partially foot the bill for the mass demobilization effort, but last month, the Senate passed a measure that would prevent U.S. assistance if the new law's perceived shortcomings were not addressed.

The law requires prosecutors to bring charges within 36 hours after receiving statements of confession, and it limits investigations of crimes to 60 days. It also requires that criminals be charged in Colombia if they confess to crimes committed there. Human rights groups say that only those fearing extradition will admit to crimes, simply to take advantage of light sentences and to avoid punishment in the United States.

Paramilitary groups are believed responsible for 12,999 killings since 1996, according to a report this year from the Colombian Commission of Jurists. They are also believed to control about half the cocaine reaching the United States.

"Commanders convicted of atrocities or other serious crimes, such as drug trafficking, will get away with sentences little longer than two years, probably in agricultural colonies," a highly critical report from the U.S.-based group Human Rights Watch stated recently.

The U.S. government has been generally supportive of the law, maintaining that Colombia should be given the freedom to pursue its own solutions to end 40 years of civil strife. U.S. diplomats have been reluctant to criticize the law directly, but they have called on the Colombians to limit concessions for the most serious crimes.

"Alvaro Uribe has transformed Colombia," R. Nicholas Burns, the undersecretary of state, said in a speech in Washington last month. "The discussions and policy differences we are having now are the result of success and our desire to build on that success."

Colombian officials say the law's faults have been simplified and overblown, but they concede that some compromise was necessary to make progress in a conflict that has proved intractable. The new law, they say, opens the door to talks with the top commanders who have driven the conflict.

"There are people who want me, immediately, to put the leaders of the paramilitary groups in jail, put them in handcuffs and show them to the world," Peace Commissioner Luis Carlos Restrepo said in a speech in Bogota this month. "I tell them, honestly, that if I were forced to do that, this peace process could go no further."

The Colombian government has not made a specific request for funding assistance to the United States for the demobilization plan, but members of the Senate appropriations committee said Washington expects to pay a significant portion of the program, estimated to cost between $80 million and $200 million.

Several U.S. senators corresponded with Uribe's government as the law was being drafted, urging officials to ensure that extradition opportunities be preserved and that paramilitary participants be required to divulge the financial and organizational structures of their groups.

"It's very disappointing. The senators' concerns have been largely ignored," said Tim Rieser, foreign policy aide to Sen. Patrick J. Leahy (D-Vt.). "President Uribe is popular, and one would have thought that they would write the law so it would achieve its goals."

Since the peace process began two years ago, more than 8,000 paramilitary members have demobilized. At a ceremony in Bogota Friday, 24 former paramilitary and guerrilla members received diplomas for completing training programs designed to prepare them for unarmed security jobs.

But critics said that because the law does not provide for the seizure of assets, the paramilitary groups will be able to recruit replacements with the promise of cash.

The government said the law could eventually apply to leftist guerrilla organizations -- including the Revolutionary Armed Forces of Colombia, or FARC -- if they also decide to disarm collectively. But the law's concessions have prompted some to worry that these groups could gain the upper hand in future negotiations.

"The guerrillas must be very pleased to see how lenient this law is," said former president Gaviria. "They can ask for more."

Paramilitary fighters from the United Self-Defense Forces of Colombia laid down their arms in a demobilization ceremony this month in Cristales, northwest of Bogota. Under a new program, they are to reenter legitimate society.