A walkout by Boston University professors, students and nonacademic staff members over charges that the trustees here "betrayed" the faculty union crippled the school's operations today in the first strike of its kind in a major private university.

Boston University's faculty, the first in the country to embrace trade unionism in a major private university, called the strike Wednesday night after the trustees reneged on an agreement made by both sides during arduous contract negotiations last week. Professors here vowed to stay out till the contract is signed.

The nation's forth largest private university remained open but few classes were held on the torpedo-shaped Charles River campus, now awash in hundreds of hand-painted placards decrying the "tyranny" of the administration and demanding collective bargaining contracts for the school's workers.

Students joined their professors and members of the still unrecognized clerical and librarians unions in a ring of pickets in the cold, damp air, peacefully blocking entrance to classes.

The faculty union at Boston University - poorly endowed and dependent on tuition for economic survival - is being watched by many of the nation's 1,500 private universities, caught in the fiscal crunch between higher costs and declining enrollment on one side of the ledger and stiffer demands for salary increases and job security on the other.

The Boston University administration is contesting the faculty union before U.S. Supreme Court, but it was ordered to bargain with the educators by a federal court here last fall.

"The serious test is: are they going to help us make perfectly reasonable savings in efficiency that are fully consistent with educational quality, but will make it possible for us to give them higher salaries," said university President John R. Silber.

The outspoken and colorful university president was convinced the faculty had passed the test. After months of negotiations, court battles and National Labor Relations Board hearings, he recommended a three-year contract boosting salaries by more than 30 percent and providing professors a greater role in academic appointments, tenure and hiring and firing decisions.

The trustees labor committee, intensely involved in negotiating with the faculty union from the start, also recommended that the full board of trustees ratify the contract.

The professors, assured by the administration in an emotional handshaking ceremony with Silber that an agreement was set, ratified the contract by a 252-to-17 vote earlier this week.

But the trustees returned with only "conditional" approval and voted overwhelmingly to seek clarification on several points in the contract.

Robert Bergenheim, a trustee, claimed that the trustee's decision not to reject the contract altogether was an act of good faith.

But Professor Fritz Ringer, president of the union, said, "Their 'clarifications' were really substantial changes."

The faculty union represents about 440 of some 850 full-time professors, those who carry most of the teaching load here. The remainder of 2,200 teachers, many of them part-time instructors, are not represented.

The faculty's action here, meanwhile, has received support from two other unions, the clerical union, which represents close to 800 employes, and the librarians union, with about 25 members. Other unions from both on and off campus generally observed the picket lines today, as did many students.