Word from the Soviet Politburo that a delegation of the U.S. House members could visit a secret, controversial Soviet radar facility last weekend came a scant 10 hours before the trip and apparently astonished Soviet officials at the site as much as it did the Americans who went there.

After arising at 3 a.m. Saturday in Moscow at the end of a week-long journey, the nine-member U.S. group took a chartered Aeroflot jet and then two helicopters 1,850 miles to the radar facility near the east-central Siberian town of Krasnoyarsk in a region of the Soviet Union previously off-limits to westerners.

Several members of the group said the unprecedented nature of the visit quickly became apparent when they learned that only one of the palpably uneasy Soviet officials and engineers at the radar site had ever met a westerner -- in 1945, at the close of World War II.

"We participated in and witnessed an extraordinary gesture by the Soviet Union," said Rep. Bob Carr (D-Mich.), who with Reps. Thomas J. Downey (D-N.Y.) and Jim Moody (D-Wis.), helped prod the Soviets into taking a substantial military risk in hopes of achieving uncertain diplomatic and political gains.

Since U.S. reconnaissance satellites first observed construction of the Krasnoyarsk radar in 1983, it has loomed large in the Reagan administration's charge that the Soviet Union is laying the groundwork for deployment of a nationwide defense against ballistic missiles, which is banned by the 1972 Antiballistic Missile (ABM) Treaty.

Citing treaty provisions that require such radars to be constructed near the periphery of U.S. and Soviet territory, not hundreds of miles inland, as at Krasnoyarsk, the administration has demanded repeatedly that the radar be torn down and used Soviet repudiation as a symbol of the country's disrespect for arms agreements.

The visit by the members of Congress and aides showed that the Soviets have not taken the administration's advice. Although the construction site had only the small Soviet host group and some plainclothes security personnel milling about on Saturday, the U.S. delegation was told that more than 1,000 laborers are working to prepare it for operation in two years.

But the visit also provided new evidence that the Soviets are eager to settle the radar dispute, and offer an example of the country's new willingness to extend glasnost from domestic politics into military policy. Several of the senior Soviet officials who accompanied the group went so far as to hint that starting the construction had been an enormous mistake, the visitors said.

The contrast between this approach and the Soviet Union's past behavior was brought into sharp relief when the U.S. delegation stopped in Omsk, a city in western Siberia, to refuel their Tupolev jet en route to Krasnoyarsk. Directing their cameras at the brightly painted shack that served as the airport control tower, the legislators were upbraided by Aeroflot officials who said the taking of photographs was prohibited for security reasons.

Yevgeni Velikhov, vice president of the Soviet Academy of Sciences who was credited by the delegation for obtaining Soviet leader Mikhail Gorbachev's support for the visit, apologized and called it an example of the "old thinking." Once at the guarded radar installation, he and Soviet Lt. Gen. Voltaire Kraskovski, an official of the Soviet air defense command, permitted the delegates to take more than 1,000 photos, draw detailed sketches and record several hours of videotape.

The legislators said the Soviets knew this information would inevitably be turned over to Pentagon intelligence experts as well as released to the public. Having persuaded the Soviet general to give them access to several areas of their choosing inside the huge radar complex, the legislators departed with confidence that only the toilets had been modified in preparation for their visit.

"They didn't think that we would be able to use a Siberian toilet," Downey said, a comfort consisting of a hole in the ground. So the Soviets hurriedly installed more familiar toilets in an effort to maintain good relations.

An impromptu banquet in an army tent erected at the site was catered by a nearby restaurateur, whose employes were astonished to see casually dressed American legislators devour roast suckling pig and deliver long toasts to international peace.