The Cuban government mounted a major attack on the U.S. Central Intelligence Agency yesterday, producing for the first time alleged double agents who claimed that CIA efforts to assassinate Premier Fidel Castro continued until as recently as 1976.

The charges were made before a so-called "Tribunal Against Imperialism" being conducted by 20,000 young leftists and communists from 140 nations taking part in this week's 11th International Youth Festival.

In addition to the new charges concerning a number of alleged CIA plots to assassinate Castro the double agents and other Cuban prisoners who testified yesterday also made new allegations related to the assassination of President Kennedy.

The Cuban government is also expected to release a 24-page document shortly called "CIA, Cuba Accuses" detailing Havana's charges that the CIA has "obscured and twisted information concerning the death of John F. Kennedy."

The decision to air these charges at yesterday's tribunal may have been designed, in part, to steal the thunder from the U.S. House Assassinations Committee, which sent a delegation here in April to pursue various questions about the Kennedy assassination.

The House panel is planning to hold public hearings on its JFK inquiry next month.

The Kennedy assassination allegations were over-shadowed here yesterday, however, by testimony that plots to kill Castro continued until only two years ago, despite CIA claims that any such activities were discontinued in 1965.

This claim was made by several double agents, including Nicholas Alberto Sirgado Ros, who said he acted as a double agent for 10 years until 1976.

Sirgado, who said he was recruited by the CIA during a visit to London in 1966, said the CIA had trained him to plant a microphone in the offices of Osmany Cienfuegos, who holds the key job of secretary to Cuba's Council of Ministers.

In 1976, Sirgado said he was asked to provide an itierary of a visit to Angola by Castro - a request interpreted here to indicate that the CIA might have been planning to assassinate the Cuban leader while he was on that trip.

Sirado said he passed carefully prepared misinformation to the CIA, and later that year, received a letter of congratulation and a wrist watch from Secretary of State Henry Kissinger.

Another Cuban, Jose Fernandez Santos, said a further threat on Castro's life was expected in Mexico in 1976.

Two men, whom he identified as Francisco Manuel Camargo Saavedra and Patricio Sanchez, scouted Mexico City's airport and a downtown momunment as possible sites for the assassination, Fernandez said.

A succession of five former double agents provoked strong emotional responses with stories of how they had succeeded in Sabotaging CIA plots to kill Castro.

One of the self-professed double agents, Abel Haidar Elias, testified that on one occasion, he had been given a powerful rifle to pass to a chosen assassin.

When the agent told the tribunal that he had never delivered the weapon, the audience gave him a standing ovation.

The panel also heard a variety of claims regarding the Kennedy assassination.

The most dramatic testimony came from Eusebio Azcue Lopez, who was consul at the Cuban Embassy on Sept. 27, 1963 when a man claiming to be the Lee Harvey Oswald requested a visa to travel to Cuba.

Azcue said "the man I saw on TV being killed by Jack Ruby, in no way looked like the man I had seen three months earlier."

The CIA photos of Oswald presented to the Warren Commission "were also not of the man I had seen," in Mexico," Azcue said. He told the tribunal he reported this immediately to Raul Roa, then Cuban foreign minister.

Rolando Cubelas Secades, now serving a 25-year-prison term also the CIA's claim that he was a double agent saying. "This is completely false, a perfidious lie."

Cubelas, who has already served more than 12 years in prison for "crimes against the state," appeared emotional as he left jail yesterday for the first time. The man who is said to have gave CIA code name AM LASH told the tribunal he worked only for the DIA between 1961 and his arrest in 1966.