Thirty-one alleged drug traffickers, including a co-founder of the notorious Medellin cartel and a high-tech smuggling magnate, were arrested today in an international dragnet that officials described as one of the biggest blows ever against the drug trade.

Colombia's national police chief, Gen. Rosso Jose Serrano, pledged that the main suspects will be extradited to the United States, a promise that U.S. Attorney General Janet Reno welcomed in Washington. No Colombian national has been extradited to the United States to stand trial since 1991.

The suspects belonged to the world's largest current drug-trafficking and money-laundering network, linking Ecuador, Colombia, Mexico, the United States and Europe in an enterprise responsible for exporting an estimated 25 to 30 tons of cocaine per month to international markets, Serrano said.

The catch included Fabio Ochoa, who with two of his brothers began the now dispersed Medellin cartel in the 1980s, and Alejandro Bernal, alias "Juvenal," whom authorities described as the brain behind a sophisticated conglomerate that laundered money and used encrypted telephone equipment and the Internet to coordinate deliveries and move profits.

Serrano described a year-long, multinational investigation that exposed the network as "the most impressive, the most important" operation ever conducted by Colombian authorities and their colleagues in the United States and other Latin American countries. Most of the arrests were made in predawn raids in the Colombian cities of Bogota, Medellin, and Cali, although at least one suspect was seized in Mexico, Serrano said.

Agents from the Drug Enforcement Administration and FBI also participated in the investigation, dubbed Operation Millennium, he added. The suspects were indicted last month by a grand jury in Miami. "The principal members of this organization will be extradited to the United States," said an announcement from Serrano's office.

Attorney General Reno, at a news conference in Washington, expressed satisfaction at Colombia's resolve to proceed with the extraditions, which has long been a touchy subject here. "I know after my conversation with President [Andres] Pastrana this morning and my conversation with officials when I was in Colombia that we will have this cooperation," she said.

If convicted of money laundering and other drug charges, the Colombians could face life sentences in U.S. prisons -- an eventuality feared by Colombian drug lords.

Authorities charged that Bernal and Ochoa operated from Colombia, shipping drugs to Mexico, where other traffickers -- whose arrests were announced last month in an investigation called Operation Impunity -- shipped them on to the United States. "Bernal no doubt thought that this arrangement insulated him from U.S. law enforcement, but this morning he found out that it does not do so," said the acting DEA administrator, Donnie Marshall.

Colombia has not extradited any citizen to the United States since it changed its law eight years ago to forbid extradition after a terror campaign masterminded by a Medellin cartel kingpin, Pablo Escobar, who was killed in 1993. But the extradition law was revived Dec. 17, 1997, and can be applied to anyone accused of crimes committed since that date.

Before today's arrests, the United States had begun the lengthy extradition process for 11 other suspects wanted for various drug trafficking charges, a U.S. Embassy official said. All have been arrested and the United States has fulfilled the requirements for soliciting extradition of 10 of them, he said. The decision of who might be extradited first -- and when -- rests with Colombia's Constitutional Court.

U.S. officials said the absence of extraditions from Colombia, the world's leading exporter of cocaine and a growing source of heroin, has been an issue of contention between the two countries. In spite of this irritant, the U.S. Congress has been laboring over a massive new aid package to help the Colombian military in its fight against the drug traffickers and their allies among leftist rebel groups that control some 40 percent of the national territory.

Against that background, Serrano hailed the intensive, multinational Operation Millennium as a model for the war on drugs. "Narco-trafficking has to be fought like this: with a lot of international responsibility," he said at a news conference at police headquarters in Bogota. "Narco-trafficking can't only be fought exclusively in only one country."

Police officers then paraded many of the defendants, handcuffed to one another in a line, in front of reporters and television cameras.

Arguably the biggest catch was Ochoa, one of three brothers who founded the Medellin cartel that became the world's most powerful drug trafficking organization during the 1980s. Taking advantage of a generous plea bargaining system, the Ochoa brothers surrendered to Colombia's judicial system in late 1990 and received light prison sentences. Fabio Ochoa, who police said is now 45, was released in 1996.

A short, slim man, he stood at the end of the line wearing a leather jacket and blue slacks. While others concealed their heads with their jackets, Ochoa gazed expressionless at the media throng, a tired look on his face.

Staff writer Lorraine Adams in Washington contributed to this report.