Taylor Swift testifies: "He had a handful of my ass. It happened to me. I know it was him."

Singer Taylor Swift has taken the witness stand during the trial of former radio host David Mueller, who is accused of groping the star during a photo opportunity in 2013.

Appearing in a federal court in Denver, Colorado, on Thursday, 27-year-old Swift told an eight-person jury that Mueller’s act was “very intentional”, CNN reports.

“It was horrifying and shocking and we had never experienced anything like [it],” she said of the moment Mueller inappropriately touched her during a backstage meet-and-greet in June 2013.

The pair were posing for a photo together, when the 55-year-old allegedly “lifted her skirt and groped her.”

He was removed from the concert immediately, and Swift’s management alerted his employer. He was fired two days later.

The radio host is now suing the star, her mother and his former employer, claiming he was “falsely accused”. He is seeking AU$3.8 million in damages.

Swift is counter-suing for assault and battery, and the two cases were merged for trial, which began this week.

Swift said she continued on with the photo opportunity after the assault, posing with fans, but said the incident "switched off my personality".

During questioning, Mueller's lawyer, M. Gabriel McFarland, suggested that Swift could have taken a break after the alleged grope.

"And your client could have taken a normal photo with me," she responded.

"I'm not going to allow you or your client to make me feel in anyway that this is my fault because it isn't.

"I am being blamed for the unfortunate events of his life that are a product of his decisions and not mine."

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She was also asked why, in the photo that many believes shows the assault in action, the front of Swift's skirt is not ruffled.

"Because my ass is located on the back of my body," she said.

She was also asked if she was critical of her bodyguard for not stepping in to stop the alleged assault.

"No, I am critical of your client for sticking his hand under my skirt and grabbing my bare ass," she said.

"It happened to me. He had a handful of my ass. It happened to me. I know it was him," she added when asked how sure she was that it was Mueller who had groped her.

"I didn't need a picture. I could have picked him out of a line of a thousand.

"You can ask me a million questions about it and I'm never going to say something different. I never have said anything different."

Taylor Swift -- The 'Sexual Assault' Photo I Wanted to Keep Secret (PHOTO) https://t.co/qT5FRZLo4T

— TMZ (@TMZ) November 12, 2016

The singer's testimony comes a day after her mother, Andrea Swift, took the stand, telling the court the incident "made her want to vomit".

She said she became very upset when her daughter said that a man had allegedly "grabbed her rear-end" during the meet-and-greet.

She said she was bewildered by her daughter's actions directly following the alleged assault: she thanked Mueller and his girlfriend for posing for the photo.

"It was just destroying her," Andrea said.

"Why did I teach her to be polite?"

Taylor Swift is suing for just $1, and has owed to donate any potential monetary awards from the trial will be donated to organisations "dedicated to protecting women from similar acts of sexual assault and personal disregard."

If you or someone you know is in need of help, please call the National Sexual Assault, Domestic and Family Violence Counselling Service on 1800 RESPECT.