A key witness in the sex-trafficking trial of the music producer Sean “Diddy” Combs took the stand on Tuesday. The rapper’s former girlfriend, Cassandra Ventura, wept as she told the jury about the physical and sexual abuse she claims she suffered over the course of their relationship that lasted more than a decade. Ms. Ventura also told the jury Mr. Combs made her carry a loaded gun. The defense is expected to cross-examine her on Wednesday afternoon. 

“I remember we took mushrooms and I was really, like, high and I was handed the gun to just hold in my bag. But I was just freaking out the whole time that it was going to go off. I didn’t — I hadn’t used or learned to operate a gun at that point,” Ms. Ventura testified at a federal courthouse on Tuesday as she remembered a night in Los Angeles when she went to a nightclub with her then-boyfriend, Mr. Combs.  

Ms. Ventura, 38, is a key witness in the trial weighing charges against Mr. Combs, 55, which began with jury selection last week. For 11 years, between 2007 and 2018, Ms. Ventura was involved in an on-off relationship with the music producer, who is charged with racketeering conspiracy, sex trafficking, and transportation to engage in prostitution, and faces a maximum of life in prison if convicted. Mr. Combs has pleaded not guilty to all the charges.   

In 2018, Ms. Ventura filed a civil lawsuit against her Mr. Combs, accusing him of rape and years of physical abuse, and even though the lawsuit was settled in a day (Ms. Ventura is said to have received about $30 million), it is credited for laying the groundwork for the prosecution. 

The former couple had not seen each other, the defense said during its opening statement on Monday, since their final break-up in 2018, and it appeared that Ms. Ventura, who is eight-and-a-half-months pregnant,  avoided all eye contact with the defendant during her testimony, while Mr. Combs turned to look at her as she entered the courtroom. 

Ms. Ventura has two children with her husband, Alex Fine, a bull rider, and the couple is expecting their third. On Monday, the defense had asked the judge to conceal Ms. Ventura’s very visible pregnancy from the jury, because a pregnant woman, as the lead defense attorney, Marc Agnifilo, phrased it, has “a prejudicial quality.”

“Pregnancy is beautiful and wonderful. It also is a source of potential sympathy,” Mr. Agnifilo told the judge, and requested that the witness be seated on the witness stand before the jury entered the room. 

An assistant U.S. attorney, Maurene Comey, objected to the request, calling it “deeply inappropriate.” The judge questioned the defense on the argument and said he did not know that there was “any support anywhere in the history of American jurisprudence” for this request. When the defense was unable to provide any legal basis for the request, the judge denied it.  

During her hours-long testimony, Ms. Ventura, who wore a turtleneck and a tight brown dress, often placed her hand on her belly, caressing her unborn child. Mr. Fine was sitting in the gallery. In the morning, the defense had asked the judge to ban him from the courtroom, arguing that it may need to call him as a witness.

One of the defense attorneys, Anna Estevao, told the judge, “Ms. Ventura began her relationship with her husband while she was still in a relationship with Mr. Combs, and there is an overlap between the two of those relationships. It also relates to our cross-examination related to her accusations about rape during the course of both of those relationships.” 

The defense had no issue with Ms. Ventura’s brother being in the courtroom, nor did it object to the presence of her cousin. But, once the judge dismissed the jury to address the issue, another defense attorney, Teny Geragos, said the defense may need to call her husband for “impeachment purposes.”

“The reason for this — and of course we do not know what her direct testimony will be at this time — but if the government elicits any allegation about any alleged rape by Mr. Combs in the summer or fall of 2018, I could see that there could be a possibility, based on how cross-examination goes, that we may need to call him on our case for impeachment purposes,” Ms. Geragos said. 

The defense also wondered if Ms. Ventura may testify about threats she has claimed the defendant made against her now-husband during the supposedly overlapping relationships. Lastly, the defense cited text messages Mr. Fine sent to Mr. Combs during the time that lawsuit between him and Ms. Ventura was settled for a reported $30 million.

Ms. Geragos, the daughter of a celebrity defense lawyer, Mark Geragos, who also represents Mr. Combs’s mother, said that after Ms. Ventura began to demand money from Mr. Combs, and prior to her filling her lawsuit, Mr. Fine “sent several very threatening text messages to our client saying that he would like to beat the F word out of his old — just several words that I will not repeat on the record.”  

The judge ruled that Mr. Fine was allowed to remain in the courtroom until the questioning about the rape allegation.  

Ms. Ventura, a former model and singer, met Mr. Combs when she was 19 years old and had just released a hit record. She alleges that the defendant coerced her into participating in debasing sex parties that he called “Freak Offs,” during which she had to engage in sexual acts, often with male escorts, and under the influence of drugs, while Mr. Combs watched and pleasured himself. 

On Tuesday, Ms. Ventura described, in detail, how Mr. Combs directed these alleged “freak offs” with an almost obsessive precision, dictating how she had to paint her nails (either in white or a French manicure) and how the participants had to always rub baby oil all over their bodies. One time, he even had an inflatable pool filled with bottles of Johnson & Johnson Baby Oil and other lubricants. Mr. Combs also asked, she testified, that the oil be heated and applied “every five minutes,” because he wanted the skin of the participants to be “glistening.’’ 

“You need to be shining,” Ms. Ventura said she was told by Mr. Combs. 

But the baby oil was the least of her problems. Ms. Ventura said she and the male escorts were also directed to take drugs, so they could stay awake and perform the sexual acts for hours, sometimes for as long as four days. Ms. Ventura told the jury that she often needed several days to recover from the drugs, the dehydration, and the sleep deprivation.  

“The freak-offs became a job where there was no space to do anything else but to recover and just try to feel normal again,” Ms. Ventura said on Tuesday.

She explained that she first started engaging in these sex marathons about six to nine months after she and Mr. Combs had begun dating, because she was in love with him and wanted to make him happy. “When you’re in love with someone you don’t want to disappoint them,” she testified.

But the “freak offs” grew more and more demanding, and though the defense argued that her participation was consensual, she told the jury on Tuesday that it was not. She claimed that when she voiced the idea that she did not want to participate anymore, Mr. Combs would threaten to ruin her career and publish video recordings he made of her sexual encounters with the escorts. 

She described the sexual acts, which she called “humiliating,” in graphic detail. One time, she said, Mr. Combs told an escort to urinate into her mouth, and she choked on the urine.

“I mean, just as humiliating if anybody were there to see it. It was disgusting. It was too much. It was overwhelming. I choked. There couldn’t have been anything on my face that was reading that I wanted to be doing that. I was kind of just laid on the floor in a position that I couldn’t easily get out of,” Ms. Ventura testified.

Ms. Ventura also spoke about the physical abuse she said she suffered during those years, testifying what happened when Mr. Combs would become enraged.

“His look would just change over,” Ms. Ventura said. “He would just become a different person. I wouldn’t know what would happen. … The best way to describe it is, his eyes just go black. … The version that I was in love with is no longer there.”

When the prosecution asked if Mr. Combs kept guns in his house, Ms. Ventura said she had seen handguns. When the FBI raided Mr. Combs’s estates in Miami and Los Angeles, the indictment states that “law enforcement seized firearms and ammunition, including three AR- 15s with defaced serial numbers, as well as a drum magazine.”  

Ms. Ventura said she was asked to carry a loaded gun one time, when she visited a nightclub with Mr. Combs, even though she does not appear to be not licensed to do so, and that she was terrified the gun would go off in her purse. She said she had “no idea” why it had been given to her. 

In 2001, while Mr. Combs was dating Jennifer Lopez, he was involved in a nightclub shooting. Three bystanders were injured, including one woman who was shot in the face. A jury found Mr. Combs not guilty of the attempted murder and gun possession charges, but his protégé, a young rapper known by his stage name Shyne, was convicted and served nearly nine years in state prisons.

Shyne, who is a citizen of Belize and was a green card holder, was deported from the United States after his release. He has insisted that he fired his gun into the air and acted in self-defense, and was the “fall guy” for Mr. Combs. The woman who was shot in the face, Natania Reuben, claims to this day that it was Mr. Combs who shot her, which he denies.

Ms. Ventura’s testimony will resume on Wednesday. The defense is expected to begin its cross-examination in the afternoon.