In what appears to be an embarrassing mistake, U.S. Customs agents boarded a Soviet airliner Tuesday, seized some common electronic equipment they believed to be defense related, and created an international incident.

The Soviet government accused the agents of using brute force and filed an official protest contending that "terror and banditism have been elevated in the United States to the status of official policy." U.S. officials defended the legality and conduct of the search and seizue, but were hard pressed to explain the basis for the sudden action or who authorized it.

A Customs spokesman said that 11 Customs and seven FBI agents boarded Aeroflot Flight 318 at Dulles International Airport outside Washington about 5 p.m. Tuesday "acting on information that the flight carried certain high-technology, defense-related items."

Seized were eight dosimeters for measuring radiation, spare parts for chemical control apparatus at a fertilizer factory and some aircraft navigation equipment being modified by the Norden Systems Co. of Norwalk, Conn., Valentin Kamenev, press counselor for the Soviet embassy, said at a late afternoon news conference. None of these items appeared to be defense related.

The Customs spokesman said the equipment was being examined by Commerce Department experts to determine if any export laws had been violated. The items will be returned if no violations are found, he said. The laws restrict the types of high-technology equipment that can be exported, especially to communist countries.

Kamenev said the seized electronics items had "for several days been in the custody of Customs, which had ample opportunity to examine them." All three boxes of equipment had been cleared by Customs earlier that afternoon, he said.

"This testifies to the fact that the U.S. government deliberately wanted to create a provocation," Kamenev said. He also said that some luggage of diplomats was ripped open with knives and searched in violation of diplomatic regulations. Two pieces were stolen, he said.

U.S. officials tried to downplay the diplomatic furor touched off by the incident. "This has no connection with our foreign policy. It was not directed by the Department of State," White House spokesman Larry Speakes said.

State Department spokesman Dean Fischer denied that any diplomatic rules were broken by the agents conducting the search. "We understand that Customs had reason to suspect that certain outbound cargo on the Aeroflot flight might not possess a validated export license." He said State first learned about the impending search about an hour before it took place.

A spokesman for Norden confirmed the Soviet official's description of the navigation equipment seized. "The papaerwork on it was in perfect order," he said. "It was commercial equipment that was properly licensed."

William Rutman, the Customs agent in charge of the operation, declined to answer questions other than to say, "This was a Customs operation. That's all I can say." James Parker, a Customs spokesman, called the Soviet description of brute force "absurd." He said the search was conducted on the basis of leads developed jointly with the FBI. An FBI spokesman said the bureau first learned of the planned search when Customs called just before the trip to Dulles and invited its agents along as "observers."

At 4 p.m., Sharon Connally, director of the enforcement division of the International Trade Administration at Commerce, said that a statement was being prepared describing the department's role in the incident.No statement was issued. One Commerce official noted that the department wastn't told about the search until 20 minutes before it began.

By the end of the day State, Customs and Commerce officials were referring reporters to each other's departments, although it seemed that Customs was the lead agency in initiating the search. One State official said, "It wasn't a screw-up in the sense that the Customs Service always has the right to inspect export cargo at any time. The question is whether the Soviets were trying to pull a fast one on something they couldn't have gotten out anyway.

"Customs made the decision to detain the plane and cargo. We handle the diplomatic flak."

Areoflot planes have been the focus of other incidents in recent years. Early last year, baggage handlers refused to load Aeroflot flights in New York to protest the Soviet invasion of Afghanistan. In 1979, U.S. authorities detained an Aeroflot plane in New York until satisfied that the wife of a defecting Soviet ballet star wasn't being forcibly returned to Moscow.