Published Thursday, 15 May 2014
Rolf Harris denies all charges. (© Getty)
The woman, giving evidence from behind a screen at Southwark Crown Court, said she was working as a waitress at the event in Cambridge when she was 13 or 14 in 1975 or 1976.
She claims the incident happened after Harris was crouched on all fours barking at a dog while "playing up" to a crowd.
"There was a small semi-circle of people around him laughing and Rolf Harris was playing up to the crowd," the woman said.
She said she felt "awestruck" at seeing the celebrity, who she recognised "instantly".
The woman, who is now 52, told the jury of six men and six women that Harris put his arm around her, and moved his hand up and down her back, before squeezing her bottom.
"It was quite a firm hold. I just stood there. I couldn't believe what was going on, this famous person putting his arm around me.
"To start it was a very nervous but a good feeling, however his hand then moved and his hand went up and down my back and his hand went over my bottom and it was very firm."
She told the court she had her back to the white marquee where the event was taking place, and so the crowd could not see what was happening.
The woman said: "I was too young to really understand the term but it was basically like groping. It was very firm and he squeezed it a few times.
She said she was "extremely embarrassed" and moved away as soon as Harris let go of her.
The woman said she contacted the police after the revelations about Jimmy Savile in October 2012 and when it was publicly revealed that Harris had been arrested, because she wanted to back up any other alleged victims.
Earlier, the jury heard from another alleged victim who broke down in tears as she told the jury the entertainer assaulted her "out of nowhere" in a packed community centre.
The woman claims Harris molested her when she went to get his autograph at the popular site near Portsmouth at one point between 1968 and 1970.
She told the jury that when she got to the front of the queue, Harris crouched down, signed her piece of paper and then touched her inappropriately.
She said she thought the first time might have been an accident, but claimed he did it a second time "aggressively".
Breaking down, the woman, who is 52, told the court she "wasn't the same child" afterwards.
Harris is accused of a total of 12 counts of indecent assault on four alleged victims between 1968 and 1986, all of which he denies.
The trial continues.
© UTV News