Some 25,000 customers are in the dark in Vaughan according to PowerStream. Another 5,000 in Richmond Hill and 3,100 in Markham have also lost power, the utility company said Saturday afternoon.
The lights are expected to come back on between 6 p.m. and 8 p.m.
Toronto Hydro meanwhile says "pockets of outages" are affecting some 1,500 of its customers in North York.
Toronto Hydro's outage map also indicates blackouts in the west end, downtown and in parts of Scarborough.