TTC CEO Andy Byford said the subway network lost radio communication at around 6:30 a.m. ET, and the backup system did not kick in.
Trains are currently holding at all platforms. Buses and streetcars are running normally.
There will be no shuttle buses operating in place of the subway trains. Shuttle buses frequently provide the only alternative means for commuters during subway shutdowns.
TTC technicians do not know why communications failed, Byford said, and it's "impossible" to know when the issue may be resolved.
Byford said he didn't want to speculate, but a series of power surges overnight may be contributing to the problem.
"I just gave up and came home after waiting for half an hour at Pape for a westbound subway that never came," said commuter Mike Karapita.
"A lot of people are starting to get really worried about how they'll get to work and other places. The platform I was on was close to packed — and that's just one platform we're talking about."
More to come