A sports doctor accused of molesting girls while working for USA Gymnastics and Michigan State University has pleaded guilty to multiple charges of sexual assault and will face at least 25 years in prison.

Dr Larry Nassar, 54, was charged with molesting seven girls, mostly under the guise of treatment at his Lansing-area home and a campus clinic.

Nassar spent nearly 30 years as an osteopath with the USA Gymnastics program, and was already in prison after also pleading guilty to possession of child pornography.

All but one of his accusers were gymnasts. Among those who have publicly said they were victims are American Olympians Aly Raisman, McKayla Maroney and Gabby Douglas.

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Ms Raisman has been particularly vocal about Nassar's misdeeds, calling him "a monster" and blaming USA Gymnastics for failing to stop him. In the wake of his guilty plea, she released a statement.

"It is about time that Larry pleaded guilty and owned up to his actions. I am beyond disgusted that a decorated Olympic and USA Gymnastics doctor was able to prey on so many over such a long period of time," she said.

"Until we fully understand the flaws in the system that allowed this to happen in the first place - and enabled it to continue for decades - we can't be confident it won't happen again. We need more than optimistic assurances, we need answers.

"We need to take a hard, honest look at the sport's culture, governance and leadership so we can understand the problem, and come up with solutions that will make the sport safer for current and future generations."

Those comments echoed her previous public condemnations of Nassar and her country's gymnastics program.

Aly Raisman, Gabby Douglas and, McKayla Maroney all accused Larry Nassar of sexual assault. Photo / Instagram
Aly Raisman, Gabby Douglas and, McKayla Maroney all accused Larry Nassar of sexual assault. Photo / Instagram

"He's a monster. It's so disturbing to think about what he did to me and so many other girls," Ms Raisman told 60 Minutes earlier this month.

"I was just really innocent. I didn't really know. You know, you don't think that of someone ... I trusted him."

"This is so much bigger than just me. It's such a horrific thing to happen. I stand here with all the other survivors who have been abused by Larry, and other people who have been abused by anyone."

She revealed she was sexually abused by Nassar as she received treatment during the moments they were alone together.

"I would lie on the table, my hands involuntarily balling themselves into fists as his ungloved hands worked their way under my clothing," Raisman wrote in her new book Fierce.

"Treatment sessions with him always made me feel tense and uncomfortable."

Several of Nassar's accusers attended the hearing in a packed Ingham County courtroom as he pleaded guilty. Some were seen crying.

Nassar admitted to digital penetrating the victims and agreed that his conduct had no legitimate medical purpose and that he did not have the girls' consent.

The plea deal in Ingham County calls for a minimum prison sentence of 25 years, but a judge could set the minimum sentence as high as 40 years.

In Michigan, inmates are eligible for parole after serving a minimum sentence. Sentencing was set for January 12.

Dr. Larry Nassar, 54, appears in court for a plea hearing in Lansing. Photo / AP
Dr. Larry Nassar, 54, appears in court for a plea hearing in Lansing. Photo / AP

The girls have testified that Nassar molested them with his hands, sometimes when a parent was present in the room, while they sought help for gymnastics injuries.

"He convinced these girls that this was some type of legitimate treatment," Assistant Attorney-General Angela Poviliatis told a judge last summer. "Why would they question him? Why would they question this gymnastics god?"

He faces similar charges in a neighbouring county and lawsuits filed by more than 125 women and girls.

He also is awaiting sentencing in federal court on child pornography charges.

The Michigan criminal cases against Nassar followed reports last year in the Indianapolis Star about how USA Gymnastics, which trains Olympians, mishandled complaints about sexual misconduct involving the doctor and coaches.

Women and girls said the stories inspired them to step forward with detailed allegations of abuse, sometimes when their parents were in the exam room at Michigan State.

- AP