To chants of "Books, not bombs!" 14 protesters were arrested at the Library of Congress last night as they sought to force officials to keep the main reading room open after the new 5:30 p.m. closing time set by library officials to meet budget cuts.

In a dramatic confrontation, Associate Librarian of Congress Donald C. Curran mounted the platform in the middle of the huge, ornate room and asked everyone to leave. At the time, there were about 65 protesters in the room, plus members of the press. Uniformed members of the library's police force stood in strategic locations.

"It is 5:31!" Curran shouted. " . . . You are now in violation . . . Please, I advise you to leave!"

Reading from a prepared statement, Curran told those in the room they were trespassing on government property, that arrest could lead to possible fines and imprisonment.

In the highly charged atmosphere, protest leader Russell Mokhiber, 31, a lawyer with the Corporate Accountability Research Group, which he described as a Ralph Nader organization, mounted the platform beside Curran and began making his own remarks. "We will not allow President Reagan to [close down] our library!" he declared to applause.

At this point, most members of the press left and the arrests began.

By the time the dust had settled about an hour later, Library of Congress police had made 14 arrests -- three women and 11 men -- and transported them in vans to the D.C. police department's 1st District station, according to a D.C. police spokesman, Officer Shannon Cockett. All were charged with unlawful entry, a misdemeanor.

Cockett said yesterday evening that it appeared "some" of those arrested would be released with citations to appear in court later.

Many of the protesters were not arrested but simply escorted to the door by police. But others, including Mokhiber, stood their ground and were placed in handcuffs one by one.

The arrests marked a sharp change in policy for the library. On Monday and Tuesday nights, officials had allowed the main reading room to be occupied by the protesters until 9:30 p.m. -- the old closing time before the budget cutbacks. A library spokesman, Nancy Bush, had described the policy: "I think we're going to let them bore themselves to death."

On Wednesday night the library was open as planned -- the only weeknight it remains open under the new budget stringency -- and there was no protest.

Just before the arrests last evening, Bush handed out an library statement saying that "unauthorized occupancy . . . can no longer be tolerated . . . nor can [the library] allow the reading room to be used for other than library purposes or to obtain publicity for personal or political causes."

To help meet this year's budget cuts, which total $18.3 million, library officials decided to cut by a third the hours that the general reading rooms are open, and have begun reducing the library's 5,200-member staff by 300. Now the reading rooms, which used to be open until 9:30 p.m. weekdays and on Sundays, are closed Sundays and evenings, except for Wednesday.

Officials have also sharply curtailed expenses for acquisitions, preservation, services for the blind and handicapped, and other activities.