At approximately 5:30 a.m., TTC staff identified signal issues in the area of Royal York Station.
It has been determined the issues were the result of an act of vandalism to TTC equipment.
Toronto Police Service was alerted and is now investigating.
Failsafe mechanisms were in place and working, allowing subways to operate safely through the area.
In order to accommodate the investigation, the TTC has shut down subway service on Line 2 between Kipling and Jane stations as of 10 a.m.
The investigation could take several hours, during which time 40 to 50 express shuttle buses will service stations in the closure area.
Stations impacted are Kipling, Islington, Royal York and Old Mill.
TTC customers heading to and from Pearson Airport can use UP Express via Dundas West Station using a TTC fare.
TTC customers can also use GO Transit on a TTC fare during the closure.
The TTC will issue service updates as needed.
All questions related to the investigation should be directed to Toronto Police.