Brazil announced today that it will conduct a search Saturday of four detained Libyan cargo planes with or without the presence of a Libyan diplomatic representative.

A Libyan diplomat said yesterday his government would do everything in its power to prevent Brazil from searching the airplanes, which were detained last weekend at two northern cities. The pilots acknowledged being on their way to Nicaragua with cargoes of arms.

Presidential spokesman Carlos Atila said the three planes in Manaus were being moved to an isolated area of the airport prior to their unloading. Manaus Airport has been closed to air traffic, and reinforcements of troops have been sent in.

Atila said if Libya failed to respond to the urgent invitation issued yesterday to send a diplomat to witness the search, "appropriate measures" would be taken.

Libya's diplomats requested a postponement and warned that the contents of the three Soviet-built planes would be a "double surprise" for Brazil, which is a leading arms supplier to Tripoli.

Representatives from Brazil's National Security Council, the Foreign Ministry and the presidential palace arrived in Manaus last night, and the search had been expected today.

In Manaus, Libyan diplomat Husne Ashur, who is representing the 38 crew members of the planes, said, "The Libyan government will go to the last resort to avoid the violation of cargoes of its planes, and we are in talks with the Brazilian government to prevent the inspection of these planes." The crew has not been detained.

The diplomat said the episode did not justify the emphasis Brazil was giving it and warned, "If Brazil insists on violating the cargo it could receive a double surprise." He said Brazil had "many commercial interests with Libya to take such a hasty attitude."

The "surprise" could take the form of Brazilian weapons in the cargo. Brazil's Air Force minister admitted this possibility but said the weapons could also be American, Soviet or European.

Following an incident in 1979, when a Brazilian-made missile sold to Libya was used by the Palestine Liberation Organization against Israeli troops, Brazil imposed end-use safeguards. Sales have continued undiminished. Libya's Col. Muammar Qaddafi has apologized to Brazil for his country having told Brazil the planes carried medicines. There are suggestions here that some sectors of the government were aware some days ago of the possible embarrassment of finding Brazilian weapons.

The implicit commercial threat centers on a $150 million Libyan order for more than 100 Tucano trainer planes, which has yet to be confirmed.

Reports of a fifth Libyan cargo plane being detained in the south of Colombia, as it allegedly delivered arms to M19 guerrillas, were denied by the Bogota government. But Colombia's defense minister, Gen. Fernando Landazabal, said he had received foreign intelligence reports that the U.S.-built Libyan Hercules cargo plane, presently detained by Brazil with engine trouble in Recife, was destined for Colombia.

The State Department and the U.S. Embassy denied reports that Brazil had either requested or been offered CIA reports about the Libyan planes, which apparently had been tracked since their departure. But U.S. intelligence sources were reported as saying that six Czechoslovak trainer planes are also on their way to Nicaragua and that spare parts said to be aboard the planes grounded in Brazil are for the trainers.

Although the Brazilian government says the first that was known of the arms shipment was when Libyan pilots in Manaus requested takeoff permission without being able to show cargo manifests or onward route details, the official account is far from complete. Brazilian officials reportedly were waiting for the Hercules cargo plane that landed in Recife as the other planes continued to Manaus.

[A statement by the Nicaraguan Embassy in Washington Thursday said the Libyan arms were a donation "to a country that is faced with aggression" assisted by the United States and came after a plea for aid "from various countries."]