Two women, 62 and 77, died at the scene, one tourist and the driver were seriously injured, and six others received minor injuries in the accident that occurred on Olkhon, the lake's biggest island with rocky banks, at about 9 a.m. local time (4 a.m. Moscow time, midnight GMT).
"The bus went out of the driver's control and overturned," the officials said, adding a helicopter was carrying the tourists to the regional center Irkutsk at the moment, where they would be taken to hospital.
"Six injured will be placed in hospital. The other two, who received grave injuries, will be brought to the traumatic surgery and orthopedics center," emergencies officials said.
There were a total of eight tourists and a guide in the minibus.