The head of a New York musicians' union local today accused New York City Opera general director Beverly Sills of "glaring" mismanagement of the opera company and called for an investigation of the opera's financial records. Union President John Glasel declined to cite examples, saying he would reveal that information before an appropriate investigatory body.

Attorney Marvin Dicker, a spokesman for the opera company, denied Glasel's allegations. And NYCO's chief negotiator, Martin Oppenheimer, characterized them as "outrageous, slanderous and totally unfounded."

But Glasel's move injected a new and potentially explosive element to the already acrimonious labor negotiations between one of the nation's most important cultural organizations and the most militant of the performing arts unions. Members of New York Local 802 of the American Federation of Musicians have been on strike against the opera company since July 6, and the opera has canceled its current season through Sept. 15.

Glasel's call for an investigation reversed his union's earlier decision not to cooperate with a board of inquiry appointed by the New York State Department of Labor. Glasel said the union would participate if the board limited itself to mediation of the strike and to investigation of allegations of financial mismanagement.

The board of inquiry ordered the union and the opera company to a "marathon negotiation" to begin Thursday afternoon.

"When this is brought to light," Glasel said of his allegations, "we are confident that the mediators will help obtain a settlement which will not result in Ms. Sills' mismanagement being paid for by the drastic reduction in her employes' standard of living."

Dicker, the opera spokesman, countered Glasel's allegations by declaring, "The quality of management at NYCO is outstanding."