Crews from Enbridge Gas Distribution are on site monitoring the situation and are working to repair the leak, the company said via Twitter. Natural gas is currently venting safely into the atmosphere, it said.
Repairs are expected to continue into the night.
Roads and rail lines in the area are being affected by the gas leak.
As of 5:50 p.m. ET, Bellamy Road is closed from Eglinton Avenue East to Trudelle Street, and TTC buses are being diverted.
GO Train service on the Lakeshore East line is suspended between Union Station and Pickering GO, but train service continues to operate between Pickering and Oshawa.