A teargas canister thrown by an unknown individual moments after Raila Odinga's running mate Martha Karua started addressing hundreds of supporters at the Gusii Stadium in Kisii County cut shot an Azimio la Umoja One Kenya coalition party rally.
Other speakers had addressed the gathering and when Ms Karua stood up, teargas coming from near the dais filled the air, sending everyone scampering.
Ms Karua was immediately evacuated by uniformed and ununiformed officers.
Moments later, residents turned on the security officers, pelting them with stones and chairs.
Mr Odinga's wife, Ida Odinga who had earlier on accompanied Ms Karua to other events in Kisii, did not attend the Gusii stadium rally.
There had been tension at the stadium and the Capital Round About rallies with differences between outgoing governor James Ongwae and some aspirants.