Prince‘s security intervened when Diddy was starting to attack Cassie outside a party hosted by the “Purple Rain” singer … one of Diddy’s former personal assistants testified in court.

The female assistant, introduced by her pseudonym “Mia,” says she worked for Diddy as his personal assistant, and later as director of development and acquisition for Revolt Films, a media company Diddy founded.

Mia told the jury she witnessed Diddy get physically violent with Cassie on multiple occasions  … including a 2010 party hosted by Prince.

The issue … Mia told the jury she and Cassie were not allowed to go where they wanted when they were together, and that when they did something without Diddy’s permission, they would be punished in bad, scary and terrifying ways.

Mia said she and Cassie ended up going to Prince’s party, where Prince performed for partygoers on a little table. She told jurors she saw Diddy show up, and they locked eyes. She said she thought, “Oh crap,” and Cassie booked it out of the party.

She said she went with Cassie across the street to some woods or bushes, but Diddy caught Cassie and had her on the ground and started attacking her … before Prince’s security intervened.

Mia said the next day HR told her Diddy was suspending her without pay and that Diddy had said she was being insubordinate.

Asked about working for Diddy, Mia said his moods changed often. She described being targeted by Diddy in alleged violent incidents … being thrown against a wall, having things thrown at her, having an ice bucket put on her head, slamming her arms into a door … and she says he sexually assaulted her more than once.

She said one of the first times she met Diddy, he was in his underwear at his apartment, after a human resources person had left.