Police were holding several people for questioning about the deaths of the girls, who were two and three years old and were discovered yesterday in the Diepsloot area north of Johannesburg after being reported missing on the weekend.
In a separate incident, South African media reported that two other toddlers were found dead in Katlehong, east of Johannesburg. Police suspect they were poisoned.
The killings in Diepsloot fueled angry accusations from residents that police were failing to provide security. Local media said some immigrant-owned shops were attacked, exposing tensions between local community members and residents from other African countries in poor areas of South Africa where there are few opportunities.
"We condemn any form of mob justice," he said. "Destroying property, looting shops, cannot be a legitimate form of resolving challenges around policing. " Diepsloot was quiet today.
"These gruesome incidents of extreme torture and murder of our children do not belong to the society that we are continuously striving to build together," President Jacob Zuma said in a statement.
Police are investigating whether the Diepsloot killings are linked to the death of a 5-year-old girl in the same area in September, the South African Press Association reported. Police suspected she had been sexually violated and strangled.