Several legal groups in South Africa say they are concerned about harsh criticism of the judge who found Oscar Pistorius guilty of culpable homicide, but not guilty of the more serious charge of murder.

South African media say police have stepped up security for Judge Thokozile Masipa, who ruled last week that the double-amputee runner was negligent but did not intend to kill when he fired four times into a toilet door, killing girlfriend Reeva Steenkamp.

Thabang Pooe, a researcher at legal group SECTION27, said Tuesday that many "personal attacks" on Masipa had appeared on social media and that they amounted to an attack on the legitimacy of the legal system.

A statement from SECTION27 and two other groups says some remarks border on hate speech and defamation.