The electronic equipment seized from a Soviet jetliner by U.S. Customs Service Agents Tuesday was not defense related or in violation of federal export laws and will be returned to the Soviets, the acting head of Customs said yesterday.

William T. Archey told reporters that his agents acted on an inaccurate tip that the material was defense to call the boarding a mistake. After receiving the tip abut 3 p.m., Customs and FBI agents boarded the plane about two hours later, just before its scheduled departure for Moscow. The flight was delayed for several hours while the agents searched for the gear.

Other administration officials hoped Archey's explanation would defuse the incident, which triggered a formal protest by the Soviet government. The Soviets charged the United States with "terror and banditism" for authorizing the search. Archey said his agents acted "politely" and professionally" and within their rights to inspect foreign-bound cargo. He denied the boarding had been ordered by higher-ups in the Reagan administration to harass the Soviets.

Archey said two of the three cargo boxes seized were improperly labeled, but he called these technical rather than substantive violations. Warning letters will be sent to the Soviet trade exporting company and one freight forwarder as a result, he added. The boxes contained dosimeters to measure radiation, some electronic spare parts and airplane navigational equipment.

Archey noted that in San Francisco recently Customs agents discovered illegally shipped computer manufacturing equipment destined for the Soviet Union labeled as washing-machine and air-conditioning parts. He said his service will continue to conduct random inspections of cargo on outgoing flights, but said there was no special effort to pick on Aeroflot.

The Customs official said that during the inspection, his agents collected the sidearms of the fourman Aeroflot crew. They also removed some panels in the plane during the search. He denied claims made Wednesday by Soviet embassy spokesman Valentin Kamenev that Customs had possession of the boxes for days before the flight. The cargo was delivered to the plane just before takeoff, Archey said.