MIAMI, NOV. 25 -- The Medellin cartel, once branded by U.S. officials as the world's most violent and powerful drug-trafficking organization, made a $10 million contribution to the U.S.-backed contra guerrillas fighting during the 1980s to overthrow Nicaragua's Sandinista government, a former cartel leader testified today.

In startling new testimony at the trial of deposed Panamanian leader Manuel Antonio Noriega, convicted smuggler Carlos Lehder, one of the founding members of the Colombian cartel, also implicated top political figures in the Bahamas and Colombia -- major U.S. allies in the war on drugs -- in cartel activities.

The testimony creates a problem for federal prosecutors who have vouched for his credibility and used him as a key witness in their drug-trafficking and racketeering case against Noriega.

Once described by U.S. government officials as the most notorious cocaine trafficker ever apprehended, Lehder was sentenced in 1988 to life plus 135 years. He acknowledged yesterday that he was testifying for the government in hopes of winning a reduced sentence that will allow him to return to Colombia. "As a foreigner, I believe eventually I will be deported back to my country before I die," he said.

Lehder's comments about the cartel's links to the contras could prove invaluable to Noriega's lawyers, who are expected to argue that the Central Intelligence Agency deliberately overlooked and even condoned close connections between the contras and drug traffickers allegedly affiliated with Noriega.

Lehder provided no details about the cartel's alleged financial contributions to the contras and appeared to give the testimony reluctantly under relentless cross-examination by Frank Rubino, Noriega's lead attorney.

Asked whether the cartel had shipped cocaine to the United States using the Costa Rican ranch of American John Hull, a principal figure in the secret contra resupply network organized by then-White House aide Oliver L. North, Lehder said that "could have been" true but that he had no direct knowledge of it.

Then Rubino asked whether the cartel had contributed directly to the contras. "To the best of my recollection, there was some contribution to the contra anti-communist movement," Lehder replied. "It could be around $10 million."

Asked if he personally participated in the contribution, Lehder responded, "Apparently, I did."

Adolfo Calero, former chief of the Nicaraguan Democratic Front, the principal CIA-backed contra organization, immediately denounced the testimony in a telephone interview. Calero said that all of the $33 million privately raised for the contras had been accounted for in the Iran-contra investigation by independent counsel Lawrence E. Walsh and that not a cent had come from drug traffickers.

"I never heard anything like it before," Calero said here. "I don't know what credibility can be given to what a convict in jail testifies to . . . . He is the lowest form of human being."

Mary Belcher, a spokeswoman for Walsh in Washington, declined to comment on Calero's remarks. But sources familiar with Walsh's inquiry noted that it dealt with the period of October 1984 to October 1986 during which Congress banned U.S. military support for the contras. The ban was circumvented by North's resupply operation. Persistent allegations, and testimony in other drug trafficking trials, have held that private pilots hired to carry military equipment secretly to the contras emptied their planes of that cargo in Central America and carried drugs on the return trips to the United States.

Lehder did not say when the money was given, or whether it was in exchange for anything. Noriega's defense team has subpoenaed a number of Reagan administration officials, including North and former U.S. ambassador to Costa Rica Lewis Tambs.

During the mid-1980s senior Reagan administration officials repeatedly denied allegations of links between the contra and drug traffickers.

Nevertheless, Jack Blum, chief investigator in a Senate Foreign Relations subcommittee probe into those alleged ties, said today that Lehder's testimony appears similar to allegations made before the panel in 1987 by Ramon Milian Rodriguez, a former cartel money launderer.

Milian testified that, in 1983, he had funneled $10 million on behalf of the cartel to the contras at the request of Felix Rodriguez, alias Max Gomez, a former CIA agent working with the contras, who had close ties to Donald Gregg, then national security adviser to Vice President Bush and now U.S. ambassador to South Korea.

Rodriguez angrily denied the charges, and the Senate panel ultimately concluded that Milian lacked credibility after he failed a polygraph test.

Blum said Lehder's testimony raises new questions primarily because "it is against his interest" to say anything federal prosecutors do not want him to say. "This leaves the government in an astonishing position," Blum said. "He's their witness, and they can't impeach him . . . . Under the circumstances, you have to ask why would he {Lehder} put this out."

Lehder also testified he personally made five contributions totaling $500,000 to the political party of Bahamian Prime Minister Lynden Pindling. "I needed the best protection I could afford," Lehder said.

Pindling has been the subject of several drug investigations, most recently by the U.S. attorney's office in Tampa after testimony during Lehder's trial in 1988 that he took payoffs from drug traffickers. But no indictments resulted, and Pindling repeatedly has denied links to drug traffickers.

Amplifying on comments he made last week, Lehder also testified that Alfonso Lopez Michelsen, a former president of Colombia whom he described as the cartel's "political godfather," represented the cartel at a crucial 1984 meeting in Havana in which Cuban President Fidel Castro mediated a dispute between Noriega and the drug traffickers. Lopez is current chairman of the governing Liberal Party of Colombian President Cesar Gaviria.

Reached last week in Miami, Lopez called Lehder's testimony "absolute rubbish."