Concert goers in Christchurch are outraged and demanding an apology after a lewd sex act was done live on stage.

Up to a hundred people turned out for the Pretty Ricky concert at The Bedford venue in Christchurch on Saturday night.

Only twenty minutes into the act, one of the American hip hop band members picked a female audience member and performed a sex act on her.

Dr Abby Suszko was in the front row and said their group immediately left when they saw what was happening.

"We were basically subjected to something that was quite pornographic and it just made me feel sick, and wrong."

Matiu Workman was also there, and said complaints will be laid with the promoter.

"We've left feeling pretty empty, pretty hollow, very disgusted and disappointed."

Mr Workman said that after the woman was picked out of the crowd, she was subsequently told by one of the Pretty Ricky band members “no matter what happens, do not close your legs.”

He said it made his group feel extremely uncomfortable, and said "this behaviour is totally unacceptable and objectifies women whilst normalising graphic, sexual acts."