ANC Gauteng spokesperson Dumisa Ntuli on Tuesday condemned the killing of two toddlers in Diepsloot's extension one.
Hundreds of people, including many women with children on their backs, marched from the Number One area in Diepsloot, where two toddlers were found dead in a toilet cubicle on Tuesday.
Eyewitness News reported on Tuesday that police were investigating whether the girls had been raped.
"We believe that the killing of kids is senseless and immoral, it is an act of those who are insane and heartless," said Ntuli.
"We condemn the brutal killing of the two innocent children and we call upon the [South African Police Service] to act swiftly to apprehend the killer."
Gauteng Premier Nomvula Mokonyane visited the scene of the crime and reassured the victims' families that the perpetrators would be arrested.
The ANC sent its condolences to the children's families and relatives, and urged parents and communities to be extra vigilant to avoid similar killings.
The girls, aged two and three, were found dead in a toilet cubicle by a community member on Tuesday morning.
Lieutenant-Colonel Lungelo Dlamini said they were reported missing on Saturday by the children's parents.
Protests mostly died down in the afternoon after rubbish was strewn around and tyres were burnt earlier in the day. Police arrived with water cannons, which were not used. – Additional reporting by staff reporter