Stunning the audience at the Philadelphia Orchestra's concert last night, Charles Dutoit announced his resignation as artistic director of the orchestra's summer series at the Mann Center for the Performing Arts, a post he had held for 10 years.

Dutoit spoke to the audience before starting the second half of the concert. He said he had made his decision at 5 p.m., and that the players had not even heard of his plans. Dutoit gave no reason for his decision.

Orchestra president Joseph H. Kluger said that Dutoit had called him at home to inform him of the decision.

"I certainly tried to convince him to change his mind and particularly not to announce it publicly," Kluger said.

When asked whether Dutoit's decision was final, Kluger said: "If he changes his mind, we would welcome him back with open arms. "

The orchestra was already in the midst of a protracted search for a successor to music director Wolfgang Sawallisch, who has announced his intention to step down by next year. The orchestra has said that Sawallisch would stay on "indefinitely," but there is no question that it must now fill two crucial posts, not just one.

The orchestra faces this predicament even as its long-awaited new performing arts center, the keystone of Philadelphia's Avenue of the Arts plan, rises from the ground at Broad and Spruce Streets, scheduled for a November 2001 opening.

Dutoit's surprise announcement last night, at the season finale of the Mann series, brought gasps from the audience. He went on to say that he had loved guiding the orchestra for the last 10 summer seasons. The audience responded with a standing ovation as he turned to conduct Carl Orff's Carmina Burana.

Dutoit first conducted the Philadelphia Orchestra in 1980 at the Mann Center and has returned almost every season as guest conductor. He was named artistic director of the Mann Center season in 1990; at the same time, he took a similar position at the orchestra's Saratoga Festival in upstate New York, which follows the Mann season.

He had been considered a strong contender to succeed Riccardo Muti as music director of the orchestra, but the orchestra gave that title to Sawallisch in 1992.

Although Dutoit's reasons for stepping down are unclear, one orchestra board member, who spoke only on condition of anonymity, suggested last night that the expected departure of Sawallisch was a factor. Dutoit, the board member said, "has the perception that our search committee is going down a path of considering candidates for music director, and that he is not on that list of people being considered. "

Dutoit, surrounded by orchestra members after the concert, said only: "These decisions are not made overnight, but after a great many things happen. Things are changing here. "

Certainly, Dutoit, who is in his 60s, has plenty to keep him busy. He is the music director of the Montreal Symphony Orchestra, and he holds posts with both the NHK Symphony in Tokyo and the French National Orchestra.

Dutoit's contract as artistic director of the Mann and Saratoga orchestra seasons runs through 2002. Kluger said that Dutoit would be with the orchestra to conduct 10 of its 15 Saratoga concerts this summer, the first of which will be Wednesday. And he will honor his guest-conducting commitments for the orchestra's 1999-2000 season at the Academy of Music, Kluger added.

Beyond that, Kluger said, "we're not going to force anyone to conduct the Philadelphia Orchestra who does not want to conduct the Philadelphia Orchestra."