A suspected rapist was severely beaten by his nephew while being arrested by police.

William Mattson, 52, was being interviewed by detectives at the property in Conway, South Carolina, when his alleged victim's boyfriend - also his nephew - attacked.

Mattson was left with two black eyes, a bleeding lip, and cuts all over his face.

Attack: William Mattson, 52, was left with two black eyes and cuts after allegedly raping his nephew's girlfriend

He was then charged with first degree sexual assault for the alleged attack on New Year's Eve.

According to a police report read by Myrtle Beach Online, Mattson assaulted the girl in the room she shares with her boyfriend after she returned home from a party.

The nephew walked in to find Mattson on top of his girlfriend, the police report claims, and dragged him off her.

After striking him repeatedly, driving him out of the house, the nephew called police.

It was as he gave his statement to officers that Mattson returned to tell his side of the story.

Claims: A police report says Mattson attacked the girl after she returned home from a New Year's Eve party

Mattson told cops he was drinking with the girlfriend after she arrived at home on her own, before they started hugging and kissing consensually.

He claims they then went to bed, adding that she was naked and he was fully-clothed.

The nephew then launched himself at his uncle, beating him severely, the police report states.