At 3:37 p.m. EDT, Environment Canada meteorologists are tracking a severe thunderstorm that is possibly producing a tornado.
The tornado warning was first issued early Saturday afternoon and gradually moved eastward from Toronto and York Region into Durham, where it now covers Pickering and Oshawa.
This is a risky and potentially life-threatening situation, the agency said. "A funnel cloud has been observed with this storm", the weather agency says, adding that heavy rain, damaging winds and large hail are also possible.
A tornado watch has been issued for Ottawa and eastern Ontario.
Tornado warnings are issued when thunderstorms are strong enough to produce tornadoes.
Environment Canada is urging people to immediately seek cover if threatening weather approaches.
"Go indoors to a room on the lowest floor, away from outside walls and windows, such as a basement, bathroom, stairwell or interior closet".
The remainder of the GTA is under a severe thunderstorm watch.