Trains travelling in both directions are not stopping at Lawrence station and shuttle buses are operating between Eglinton and York Mills station.
CBC's Linda Ward reported from the scene at 6:30 a.m. and said workers were trying to isolate the location of the water main break.
"They have water leaking onto the platform at Lawrence station. They don't know exactly where the leak is," she reported.
"They were trying to figure out which valves to shut off. They were getting ready to start turning off those valves. That should start happening right now. That may mean no water for some in the area.
"Lots of people are arriving trying to use the subway and confused about what's going on."
Ward said it's not yet clear when regular TTC service will return to the station.