Cassie Ventura says Diddy had blackmail material on her … namely, videos from their “freak-offs” … and she says he used the footage to get her to do more freak-offs.

She testified Wednesday Diddy would hold the freak-off footage over her head as a way to hurt her, make her fearful, and coerce her into participating in sex parties with male escorts.

Cassie told the jury she had previously told Diddy she didn’t want to do freak-offs, and that on a commercial flight to New York in June 2013, he sat next to her and played her freak-off videos of her on his laptop and threatened to release them.

Prosecutors said in their opening statement Diddy threatened to release the videos if she did not participate … they are clearly trying to prove this was one way Diddy coerced Cassie into doing the freak-offs. In last year’s indictment of Diddy … feds described freak-offs as “elaborate and produced sex performances that Combs arranged, directed, masturbated during, and often electronically recorded.”

Cassie’s mentioned extortion before in her testimony, but this is the first time she’s closed the loop and said Diddy used the threat of releasing freak-off footage to get her to do more freak-offs.

She also told the jury Diddy once threatened to release the videos while she was dating someone else. She said she would have had to answer to her mother, and Diddy’s threats were horrible and disgusting.

Cassie told the jury Diddy kept the videos on his phone … and she said she sent him a text message telling him to delete the footage.

She read a text message she’d sent to Diddy in July 2013 … “Please delete the videos off your phone, too many people have access to your devices.”

Cassie testified Diddy responded, “No iCloud, all good, Ambien kicking in.”

She also told the jury she reached out to Diddy after being told there was a sexually explicit video with her in it that was floating around … and she says he had his security team intimidate her.