In a bitter manifestation of growing tension with the United States over cooperation in fighting drug trafficking, Mexico's Government-owned television network has accused an American drug agent who was slain here of having been part of a major cocaine smuggling ring and of having used his position to blackmail drug dealers and enrich himself.

A documentary program broadcast on the network Sunday charged that Enrique Camarena Salazar, who was kidnapped by Mexican police officers on Feb. 7, 1985, and found dead a month later, ''was one more drug trafficker, infiltrated in the American anti-drug agency, who betrayed his accomplice and partner Rafael Caro Quintero and for that reason was murdered.'' The network named no source for its charges, saying only that ''clear evidence'' had been uncovered ''during investigations in Mexico.''

The television program was broadcast in response to an NBC mini-series called ''Drug Wars: The Camarena Story,'' shown on three nights last week. Based on the book ''Desperados,'' the program depicted widespread corruption and complicity in drug trafficking by Mexican officials and was accompanied by news segments in which American officials expressed reservations about Mexican efforts to halt drug trafficking.

D.E.A. Is Denounced

The accusations against Mr. Camarena were accompanied by a strong attack on the Drug Enforcement Administration, the agency for which he worked. ''The D.E.A. is today as fearsome and abhorrent as the Holy Inquisition and as corrupt, or perhaps even more so, as the very narcotics traffickers it pursues,'' said the narrator of the Mexican program, who also charged that the D.E.A. possessed evidence that would establish Mr. Camarena's guilt.