MMC member Douglas Maimane was sitting on stage alongside Water Minister Edna Molewa when a water bottle was thrown at him by a brother of one of the deceased.
Maimane was hit in the face, causing his mouth to bleed.
The brother was crying and breathing heavily. His family tried to calm him as he stared at the stage, where Molewa remained seated.
Three men were killed in the township during protests over water this week.
On Monday, mineworker Osia Rahube and a photographer known as Bra Mike were shot dead, allegedly by police.
On Tuesday Lerato Seema died, allegedly after falling from a moving police Nyala. Police said he was trying to escape from custody.
Residents claim they have been without water for at least three months.
On Wednesday, Molewa promised residents of Mothutlung that water would be completely restored by Friday.