Federal agents seized a Colombian Boeing 747 jet early yesterday at Miami International Airport, two days after nearly 2,500 pounds of cocaine with a street value of about $600 million was found concealed in its cargo of Valentine's Day carnations.

The seizure of the jet, which is owned by the Colombian government airline, Avianca, was one of the most aggressive actions to date in the stepped-up U.S. effort to block movement of drugs from Latin America and other producing areas.

The U.S. Customs Service accused airline employes of knowing that the cocaine was aboard. The Colombian government promised to investigate but deplored the "publicity-oriented display" of the Customs Service. Avianca Airlines said it was a victim, not an accomplice, of the drug traffickers.

In an attempted "sting" operation, federal agents on Wednesday substituted a valueless white power for the cocaine and allowed the shipment to continue as scheduled to Montreal on an Air Canada plane. No one had been arrested by late yesterday.

To avoid raising suspicions, the Colombian jet was allowed to make another round trip to Colombia before it was seized.

The cocaine was discovered in a warehouse, officials said, by Sidney Reyes, the U.S. Customs Service district director at Washington Dulles International Airport, who happened to be in Miami for a refresher training session.

U.S. Customs Commissioner William von Raab said the $119 million jumbo jet was seized at 5 a.m. yesterday, about three hours after it landed. The jet is the largest plane ever seized by U.S. agents.

Dennis Murphy, a spokesman for the Customs Service, said yesterday that although agents have seized many commerical aircraft, he believes that it was the first seizure of a plane owned by a foreign government.

Murphy said customs agents seized the plane because they believe that "employes of Avianca . . . had knowledge that the cocaine was on board." He said the flight crew clearly should have been concerned about the presence of 2,500 pounds of unmanifested cargo.

According to a statement released yesterday by the Colombian Embassy here, "The government of Colombia is conducting an investigation and collecting facts from all sources to determine the exact origin of the narcotics seized in Miami.

"Pending the results of the investigation, the government of Colombia deplores the publicity-oriented display which the U.S. customs commissioner set up in reference to this event and which is seriously detrimental to the good image of Colombia as a committed ally of the United States in the all-out war against drug trafficking."

Pointing to a recent State Department report praising Colombia's drug enforcement efforts, the embassy said, "Our country will continue to step up efforts in this area in close cooperation with U.S. authorities."

Roberto Durana, chief of public relations for Avianca in Bogota, denied that airline employes knew of the cocaine shipment. "It is a very difficult question," he said. "We have been the victims of a chain of drug traffickers who don't hesitate to take advantage of any means to carry out their trade."

"We are talking about 5,000 or 6,000 boxes of flowers and Avianca can't open every one of the boxes to see what's inside, any more than it can open the suitcases of its passengers, since this isn't the responsibility of the airline," Durana said.

He said that Avianca officials were working with Colombian narcotics police in an "intensive investigation" and that the airline hoped "to get our plane back as soon as possible."

U.S. customs officials, acting on information provided by the Drug Enforcement Administration, discovered the 2,478 pounds of cocaine Wednesday in a shipment of carnations and other cut flowers from Bogota.

Murphy said the flowers were unloaded and stored in a refrigerated warehouse for a routine customs inspection, while the Avianca jet was loaded with new cargo and flew back to Colombia. The jet then made another round trip to Colombia before it returned to Miami and was seized yesterday.

The cocaine, which was concealed among 32 flower boxes and was believed to have been wrapped in plastic packages about the size of footballs, was found when Reyes, crawling among the flowers, inserted a probe into a box.

Reyes, who has worked for the Customs Service since 1959, said in a telephone interview yesterday that the probe is a long metal rod with a serrated end which is used to look for drugs such as cocaine or heroin.

"When you stick it into a packet containing a powdery substance, it goes in like a bullet, and when it comes out, the powder clings to the metal edges," he said.

During the search in the warehouse, Reyes said, "we had a prior game plan that if we discovered anything, we were to keep cool, not show any outward manifestation. We didn't want to alert all the people who were around the warehouse." After he found the powder, Reyes said he "eyeballed the supervisor" to get his attention.

After the discovery, Customs called in the South Florida Drug Task Force, which surreptitiously removed the cocaine and replaced it with an unidentified white powder. Jim Dingfelder, a task force spokesman, said that the shipment was sent via Air Canada on Thursday night to Montreal, where the investigation is continuing with the assistance of the Royal Canadian Mounted Police.

Customs officials said that the shipment, which arrived 24 hours later than scheduled, was met by a person who was interrogated by Canadian authorities but not arrested.

The Avianca jet was seized in Miami under provisions of the Tariff Act, the Transportation Act, and the Controlled Substances Act, according to von Raab. These provisions provide for the seizure of conveyances used in the transportation, concealment and importation of controlled substances, officials said.

Customs officials said that this was not the first time drugs had been found on an Avianca aircraft. In addition to this week's seizure, a total of 5,575 pounds of cocaine has been found in 34 seizures aboard Avianca flights since April 1980, according to the Customs Service.

A customs official said the 747 will "be out of commission" for some time. Even if the airline is cleared by the task force's investigation, he said it will cost Avianca "a very substantial penalty, well over a million dollars I would guess" to get the plane back.

According to officials, the Avianca cocaine seizure was the third largest in U.S. history.

The largest seizure occurred at Miami International Airport in March 1982 when agents discovered 3,906 pounds of cocaine in a shipment of blue jeans from Colombia on a Tampa-Colombia flight, customs officials said.

The second largest occurred last July when 2,654 pounds of coca base was found hidden inside an air cargo shipment on Viasa, a Venezuelan carrier. Coca base can be converted into an equivalent weight of cocaine.