Activist Boniface Mwangi suffered a burn mark on his chest after an anti-police officer lobbed a tear-gas canister straight at him.
Mr Mwangi suffered the injury on Thursday morning while leading a demo to protest recent killings of protesters by police.
Mr Mwangi and his Team Courage protesters had assembled at Freedom Corner when anti-riot police moved in to disperse them.
He got injured in the ensuing melee.
Police later lobbied teargas at Jevanjee Garden to disperse members of the group who had regrouped there.