A man suspected of being South Africa's most prolific serial rapist has been found dead in his prison cell hours before he was to go on trial for crimes that included sexually assaulting 34 children, prosecutors said.

Sifiso Makhubo, 42, was facing a total of 122 charges for murder, rape, kidnapping, robbery, and attempted murder by infecting his victims with HIV, prosecutors said on Thursday.

He was thought to have hanged himself with a blanket in his prison cell and authorities were investigating the circumstances of his death.

The horrific nature and breadth of the alleged crimes shocked South Africa, a country inured to some of the world's highest levels of sexual violence.

In the year to April 2012, more than 64,000 sexual offences, including rape, were reported among South Africa's 50 million people. Of these, more than 25,000 were assaults on children.

In 2010, South Africa recorded 132.4 cases of reported rape per 100,000 population, by far the highest among countries that reported statistics to the UN, compared to 27.3 in the US or 2.1 in Uganda.

Reporting rates, and definitions of rape, do, however, vary sharply from country to country.