Two boys, aged 16 and 17, were arrested while they were raping a girl in Welkom, Free State police said.

"They were raping a 16-year-old girl along the railway line between Thabong and Welkom," said Sergeant Malebo Khosana.

Police patrolling in the area saw what was happening and immediately tried to stop the boys.

"One of the boys tried to run away but was caught... he was found with a homemade knife with a hard putty handle."

Both boys were taken to Welkom police holding cells.

The two will appear at Welkom Magistrates' Court soon.

Sapa