Hit by budget cuts, the Library of Congress yesterday announced it will slash the hours that its general reading rooms are open to the public by one third, reduce its 5,200-member work force by 300 and sharply curtail expenses for acquisitions and preservation.

"The effect, in a word, will be disastrous," said Librarian of Congress Daniel J. Boorstin last night.

The library's general reading rooms, a haven for scholars and bibliophiles from around the world, will open a half-hour later Monday through Saturday and be closed Sundays and all federal holidays. They will also be closed evenings during the week, except Wednesday.

Previously these public rooms -- which include the main, microfilm, periodical, science, social science, local history and genealogy rooms, and the law library -- have been open every evening until 9:30, Sundays and holidays except Christmas and New Year's.

The changes in hours -- an overall reduction from 77 1/2 to 54 1/2 hours a week -- take effect March 9.

Students and scholars in the main reading room yesterday evening said they were upset at the news.

"If you want to cut back on knowledge and free exchange of ideas, this is a good way to do it," said Susan Klaus, a graduate student in American studies at George Washington University. She said she "practically lives" in the Library of Congress and added, "I'd rather be asked to donate a dollar each time."

"The administration has made its choice as to what it supports -- defense instead of museums and libraries that serve a good purpose," said Josh Sugarmann, who said he was doing research for the National Coalition to Ban Handguns.

Another GWU student, Karen Diller, said she has to work during the day to support herself, so the elimination of evening hours will make her research in classical archeology "much more difficult."

Boorstin said in a telephone interview that the reduction in hours will have "an antidemocratizing effect" because "people with other jobs and who who want to use the library for research, and who are not gentle men of leisure, will not have the opportunity they had."

Boorstin said services to Congress will be cut and "the world of learning" generally will suffer because "thousands of volumes" that should be acquired will not be, cataloguing will be curtailed and services to the blind and handicapped will be cut.

Said Boorstin: "What troubles me about it is that the greatest nation in the world should not allow its knowledge resources to be cut off and depleted."

The library's $220.3 million budget for fiscal 1986 is $18.3 million less than last year's. Library spokesman Nancy Bush said yesterday that Congress initially cut the library's 1986 budget by $8.4 million: "There were across-the-board cuts in travel, some attrition in hiring, supplies . . . So it wasn't terribly dramatic, but it made it more difficult to absorb a new cut."

An additional $9.9 million was cut as a result of the Gramm-Rudman-Hollings budget reduction act. This was difficult to absorb, Bush said, because "we had already tightened our belts."

Boorstin said that "the law requires that the cuts be made equally on all appropriated items," so that he had "little leeway" in deciding what should be cut.

Bush said that staff reductions will be through attrition, early retirement, termination of temporary employment and hiring freezes.

"If that doesn't do it," she said, "we will have to have involuntary separation."

The new hours for the general reading rooms will be 9 a.m. to 5:30 p.m. Monday through Saturday except Wednesday, when the library will be open from 9 a.m. to 9 p.m.