MOSCOW, JUNE 8 -- Ridding the new American Embassy here of Soviet eavesdropping devices will require at least a 2 1/2-year overhaul that will cost "many tens of millions" of dollars, the head of a State Department investigating panel said here today.

James Schlesinger, a former Central Intelligence Agency director, told a press conference that the Soviets had penetrated the newly built complex by planting in precast concrete sections "a higher plateau of technology" -- listening devices not detectable by traditional U.S. X-ray scanning. They were discovered through a "new detection device," Schlesinger said.

Schlesinger said that ensuring the security of the embassy calls for "fundamental reconditioning" and "significant restructuring." If the Soviets do not cooperate in the U.S. efforts, he said, it could take "decades" to resolve the security problem.

"The number of implanted devices -- which as yet we do not fully understand -- is substantial," Schlesinger said in conference with American journalists at the end of a 10-day investigation here that was ordered by Secretary of State George P. Shultz. During an April visit, Shultz referred to "a honeycomb of listening devices" in the embassy.

"We face a rising curve of technology and technological progress with regard to . . . intrusion devices," Schlesinger said, adding that while the Soviet Union doesn't have better devices, it "has been more ingenious at exploiting" the technology.

He declined to detail his proposals, which he plans to present to Shultz and to Congress. But western diplomats here have speculated that one solution would be to add additional secure floors to the 10-story structure.

The Senate Select Committee on Intelligence recommended in April that the building be demolished and rebuilt and some administration officials have made similar recommendations. The construction was originally budgeted at $70 million but recent estimates have put the final cost at well over $100 million, not including the expense of removing listening devices.

Schlesinger, asked whether he thought the embassy would have to be demolished, said only that "we will be able to preserve a significant portion" of it.

He said that the needed overhaul pertains largely to the "secure section" of the embassy. Parts of the building where maximum security is not required could likely be preserved, he said. He held the news conference in a wing of the new embassy, which is largely completed but remains unoccupied.

Schlesinger, a Washington consultant who has served as secretary of defense and energy in previous administrations, was appointed by Shultz four months ago to head a probe into security problems at the new embassy building.

His team arrived here May 29 and has spent the past 10 days investigating the new embassy compound and talking with Soviet officials, including Central Committee Secretary Anatoliy Dobrynin.

Schlesinger said that Soviet cooperation will be needed in any restructuring, since that would necessitate a renegotiation of the 1972 agreement under which the two countries set conditions for building new embassies in the two capitals.

Under the agreement, the two embassies are to be occupied simultaneously. The new Soviet Embassy in Washington is complete, but cannot be used until the United States accepts the embassy here.

In the construction of the building, Schlesinger said, "various {Soviet} government agencies had availed themselves of the opportunity" to plant devices.

"We did X-ray everything that came on site," Schlesinger said, adding that "the Soviets have moved to the point of developing capabilities that were not detectable by X-ray devices."

"Were it not for the fact that we have a new detection device," he said, "this would have gone undetected." He gave no details on the new device.