VIENNA, Austria (AP) _ Long-range predictions that the Chernobyl disaster may cause 20,000 or more cancer deaths in the Soviet Union were based on worst-case scenarios and were much too high, some nuclear experts said.

A Soviet nuclear scientist, meanwhile, said today that the comments of experts from 50 nations at a weeklong conference reviewing the April 26 accident were ''most useful.''

''The further development of atomic energy requires many more efforts at raising safety standards and achieving a better international exchange of information,'' said Valery A. Legasov, a deputy chairman of the Soviet Union's main atomic energy institute.

Earlier this week, some Western officials attending the conference under the auspices of the International Atomic Energy Agency said up to 25,000 people might die over the next 70 years in the western Soviet Union because of the nuclear power plant accident.

But Leonid Ilyin, vice president of the Soviet Academy of Medical Sciences, told a news conference Thursday those predictions were derived from initial Soviet estimates of radiation exposure made on a ''pessimistic theoretical basis'' rather than actual measurements.

''We have carried out a wide campaign of measuring real levels (of radiation exposure in the 135,000 people evacuated from the 18-mile zone around Chernobyl), and these real levels turned out to be 10 times less than the calculated values,'' Ilyin said.

The Soviets have not disclosed any estimates of eventual Chernobyl-related deaths.

Dan Beninson, an Argentine nuclear safety official, said 2,000 would be a more accurate estimate of future Chernobyl-linked cancer deaths in the Soviet Union. He said estimates in the 20,000 range were ''nonsense, in many respects. ''That does not mean that if we have only 2,000 instead of 20,000 it is good or more acceptable,'' he said. ''It is a very bad number in any case. But it has to be put into perspective.''

The explosion and fire at Chernobyl's No. 4 reactor in the Ukraine, 80 miles north of Kiev, killed at least 31 people and sent radioactivity over much of the world.

Dr. Robert Gale, a Los Angeles bone marrow surgeon who treated some Chernobyl victims, told The Associated Press on Thursday that estimates of future worldwide cancer deaths resulting from the accident range from 1,000 to 75,000. He said health experts believe ''the truth will lie between the extremes.''

Legasov told today's concluding plenary session that Soviet scientists had ''listened carefully'' to the comments and questions of several hundred Western experts at the IAEA-sponsored conference.

''This experience has been most useful to us, as it sometimes drew our attention to things we had missed,'' he said, speaking in Russian. His words were simultaneously translated into English.

''We (Soviet scientists) had a feeling of solidarity this week rather than criticism,'' Legasov said. ''This reflects the realization that we should not reverse the technological progress of civilization by rejecting the peaceful use of nuclear power'' but instead improve the safety of nuclear energy.

Swedish Energy Minister Birgitta Dahl said Thursday, ''We know we will have another accident like that at Chernobyl within 10 years if nothing is done to increase safety standards considerably. Many of the Soviet nuclear plants would not be operating if our Swedish safety standards were applied there.''

Hans Bartsch, a Swedish government nuclear safety expert, quoted a report by his department as saying a Soviet nuclear power plant at Ignalina in Lithuania was an example of an unsafe Soviet reactor.

Helmut Rabold, an East German atomic safety official, said Soviet and Western delegates had suggested establishing an international agency, composed of civil defense and military personnel, to pool efforts against any future atomic accident.