Bill Cosby's sexual assault retrial began in chaos after a topless protester launched herself over a barricade and lunged at the 80-year-old comedian.

The woman, who previously starred in several episodes of 'The Cosby Show', was wrestled to the ground by police as Cosby arrived at a Pennsylvania courthouse.

Written across her body was "Women's Lives Matter" and the names of more than 50 Cosby accusers.

The 39-year-old actress, named as Nicolle Rochelle, was taken into custody and charged with disorderly conduct but was later released.

Rochelle said she did not have any bad experiences with Cosby herself when she was on the show but wanted to make a statement.

"The main goal was to make Cosby uncomfortable because that is exactly what he has been doing for decades to women, and to show him that the body can be aggressive and empowered," she said afterwards.

European feminist group Femen has claimed Rochelle as one of their protesters.

Cosby spokesman Andrew Wyatt praised police officers for their quick response but urged court officials to increase security.

"It's a different world. Things have changed," Wyatt said, referring to recent mass shootings and other events.

"You never know who's going to want to make a name for themselves."

Cosby faces trial on charges of drugging and molesting a woman 14 years ago. It is one of a number of claims he denies.