One of the four men whose extradition from South America in January has been linked to threats of violent retaliation against federal agents by cocaine smugglers goes on trial today at U.S. District Court, amid some of the tightest security precautions in years.

Held in an isolated cell in New York since early January, defendant Marcos Cadavid has been brought to the courthouse here for pretrial hearings in a black limousine escorted by U.S. Park Police on motorcycles, under the scrutiny of police officers in a helicopter.

Charged with conspiracy to import and distribute cocaine from 1976 to the end of 1982, Cadavid, a 43-year-old Colombian, has been chauffeured at high speed in and out of the underground garage of the courthouse under the watchful eyes of plainclothes agents packing automatic weapons.

Authorities have refused to discount threats of an attack against Cadavid or the courthouse at Third Street and Constitution Avenue NW by drug dealers' hit squads.

The defendant is to go on trial behind a security cordon characterized by courthouse veterans as even more elaborate than that mounted for the last high-profile defendant, presidential assailant John Hinckley Jr.

Cadavid, who says he makes his living as a small farmer in South America, is a figure of major consequence in the eyes of the U.S. Justice Department.

The extradition of Cadavid and three others in January marked the first use of a new treaty between the United States and Colombia aimed at cracking down on the international cocaine trade.

Officials say intelligence sources have reported that Colombian-backed hit squads have been dispatched to this country to retaliate by killing or injuring U.S. agents.

The threats "are sufficiently credible" that they are being taken seriously by U.S. authorities, said a senior law enforcement official. "It's pretty clear that the Colombians would like to take out a federal judge."

Officials refuse to say whether District Judge Thomas F. Hogan, who will preside over Cadavid's trial, has been threatened.

Special courthouse security precautions have been in effect for the past eight weeks, however, while Hogan conducted a trial for four defendants in a case related to Cadavid's alleged drug smuggling.

In court papers, federal prosecutors paint a picture of Cadavid that differs markedly from his own version of himself as a simple man of agriculture.

According to the government, Cadavid was one of two Colombians who formed a Miami connection supplying many kilograms of cocaine to an admitted Bethesda drug dealer, Lawrence G. Strickland Jr. Strickland has pleaded guilty to distributing the drugs, at large profits, in the District and elsewhere. He is expected to be the star government witness against Cadavid.

The second Colombian associated with Cadavid, Armando (The Hammer) Marulanda, is a fugitive.

Strickland first met Marulanda in early 1979 at the home of a Miami lawyer, where Strickland turned over $350,000 in cash to the Colombian, Strickland has said in court papers. "Approximately five minutes later, Marcos Cadavid entered . . . with a suitcase," the record states. In the suitcase, Strickland said, were 10 kilograms of cocaine.

About three weeks later, according to Strickland, the scene was repeated. Strickland said he gave Marulanda the balance of $140,000 he owed on the first transaction, along with an additional $300,000 in cash. " . . . And a few minutes later Marcos Cadavid arrived with a suitcase containing 10 more kilograms of cocaine," the record states.

A short time later, in the spring of 1979, the group met a third time, the court papers said. The buy: 20 kilograms of cocaine. Strickland said he and a friend gave Marulanda and Cadavid $500,000 in cash as a down payment on the drugs.

From then until September 1979, Strickland said in his statements, he and his associate "regularly purchased 20 kilograms of cocaine from Mr. Marulanda and Mr. Cadavid" in Miami.

Finally, in the summer of 1980, "Cadavid indicated to us that he was now running the drug business in Miami and that Marulanda had retired," Strickland said in the report.