The former British comedian and TV host, now 51, is accused of groping a woman's breasts before dragging her into a toilet to perform a sex act.
In her recorded interview with police, the woman said she was excited to meet Brand at the 2004 birthday party, but says within a minute of starting a conversation he had sexually assaulted her.
She alleges Brand then told her "don't be a spoilsport" while forcing her to perform oral sex in the toilet cubicle.
"He literally just grabbed my boobs, out of nowhere, just in the middle of a conversation and I just remember this weird maniacal grin, looming in front of me as he just grabbed my boobs," she said.
"I was completely in shock and was like 'what the hell are you doing? I've literally just met you'.
"Then literally he grabbed my arm and dragged me into the toilets."
In her interview with police, which was played on Monday at Southwark Crown Court, the woman recalled Brand looking "like a clown, (with) that sort of weird look in the eye" after grabbing her breasts.
Brand is on trial accused of three counts of rape, three counts of sexual assault, and one allegation of indecent assault.
He denies all the charges and says all the sexual encounters were consensual.
The woman, who was in her 20s and working for MTV at the time, told police she was assaulted by Brand at Nordic Bar in Soho in 2004.
She remembers being told that Brand – whose fame was rising as he presented Big Brother spin-off shows on Channel 4 – was due to come to the birthday party.
She asked a friend to introduce them, and said she was groped by Brand within "a minute max".
She said he "dragged me into the toilet" where the floor was flooded, and then jumped on to the closed toilet lid.
"It just happened so fast", she said.
The woman said she "froze" and "didn't know what to do" as she was forced to perform the sex act, and after the alleged rape she remembered a man knocking on the toilet door saying: "Are you OK?".
"Russell just sort of casually walked out and unlocked it and I just walked out in a daze," she told police.
"I remember this really, really clearly – he turned to me and went 'Oh, I guess we should swap numbers now', like it was just all normal, like he hadn't just forced me to do that."
The woman said she felt "numb" afterwards and walked around the bar trying to find a friend, but says she was then ushered into the back of a car – which she believes was hired by Big Brother producers Endemol – with Brand.
She recalled: "I just felt this nudge… and I looked round and he had his penis out again in the car, with these two guys sitting in the front, and he's going 'Come on, come on, suck it', and I was just like 'no'.
"He took 'no' for an answer then."
She said 2023 media reports about Brand's alleged sexual misconduct led her to come forward to police, telling the officer: "Seeing it all in the press has made me relive it all."
Brand, who was one of Channel 4's top stars in the 2000s and also had a BBC Radio 2 show, is also accused of raping a woman in a hotel room at the Labour Party conference and trying to drag another woman into a toilet at the MTV studios.
The trial continues.
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