Johannesburg - Three buses carrying DA supporters were stoned, two of them severely, in Alexandra and near the FNB Stadium in Soweto on Sunday, the party said.
Gauteng police and Johannesburg metro police were not immediately aware of the incidents.
Democratic Alliance Gauteng candidate Mmusi Maimane said in a statement that hundreds of DA members were being interviewed by police after "ANC ambushes".
The DA buses were taking supporters to a DA "We Can Win" concert at Walter Sisulu Square, in Kliptown, Soweto. The ANC is holding a rally at the FNB Stadium around 15km away.
Maimane said the driver of one of the buses was hospitalised.
"One bus was so heavily stoned that it is no longer usable."
Maimane's spokesperson Kelly Clark could not immediately give further clarity on where the incidents happened.