A Manhattan pianist claims a Metropolitan Opera conductor forced her as a young apprentice to attend a performance of “La Traviata” with him, then exposed his penis and tried to guide her hand to his lap as they sat in the audience.

The woman, who did not want to be identified but affirmed her account in a sworn affidavit, was a 25-year-old assistant conductor-in-training at the time of the alleged 2001 encounter with Joseph Colaneri, then a teacher in her program.

“I was just horrified,” she told The Post last week.

The woman said she yanked her hand away and fled the theater during intermission.

“This was my first important job in the opera world, and it nearly made me want to quit forever,” said the woman, now 41, who was an apprentice in a summer program at the San Francisco Opera at the time.

Colaneri, 62, who conducted a performance of “Norma” as a staff artist at the Met Saturday, was a visiting conductor in the student program in San Francisco in the summer of 2001. The woman had just graduated from college, she said.

She said she told her story to colleagues in San Francisco, and later New York, but their response was to keep quiet.

“Everyone told me that the professional thing to do was just to ignore it and focus on my work. I was scared, but that’s what I did,” she told The Post. “Everyone kept telling me this is how things are.”

Asked if those colleagues would corroborate her story for The Post, she said they are too afraid of losing their jobs.

Colaneri didn’t return repeated requests for comment.

Found at his New Jersey home Saturday, he refused to comment.

The woman told her mother shortly after the incident.

“She was very upset,” the mom recalled. “Her feeling was that she didn’t know if she could go on. We were heartbroken.”

The woman also told her boyfriend shortly after they began dating in 2015.

“She told me about Colaneri soon after we met, while we were both studying in Sicily,” the boyfriend said.

The woman avoided Colaneri for several years, but their paths crossed in New York, where she doubles as an opera coach and freelance musician. She freelances for the Met and, until last week, held a part-time job at the Mannes School of Music at The New School, where Colaneri heads the opera department. Her boyfriend is a rising star in the school’s opera program.

“To this day, this man is holding my noncompliance against me in the professional world, and also attempting to hurt my loved ones,” she said.

When he found out about her relationship with the student, Colaneri would ask her personal questions and make remarks about their home life.

“At first, I just thought he was just commenting on it, but it got creepy after a while,” she recalled.

Last week, she sent an anonymous complaint about Colaneri to the Met, encouraged by recent allegations against famed conductor James Levine, whom the Met suspended this month over claims he molested a teen.

The married Colaneri is one of 23 in-house Met conductors, the opera house’s Web site says. He conducts a handful of operas every year and sometimes fills in for star conductors.

Two days after she contacted the Met on Dec. 8, Colaneri tried to block her from attending a Mannes production that he was conducting at John Jay College and that featured her boyfriend, she said.

“He had told the stage crew not to allow me into the theater,” she said.

Colaneri dispatched a stagehand to find the woman, who was already in the audience.

“He was in a state of high agitation,” said a performer who heard Colaneri say he wanted the woman removed.

Two days later, she said she learned that she lost her part-time gig as an opera musician at Mannes, where Colaneri is a professor. A spokeswoman for the school says she was terminated on December 8, but refused to comment further.

A New School spokeswoman said she was “unable to comment” on employees.

A Met spokesman did not return requests for comment.

Additional reporting by Jennifer Bain