The noise from the leaking main in Kirkstall was so loud that some local residents mistook it for gushing water - it could be heard up to 100m away from the site.
One resident of Victoria Park Avenue said: "At first I thought it sounded like a plane going over because it was that loud. My wife thought it was gushing water."
The leak occurred shortly before 5pm today (Friday August 10) close to the junction of Wyther Lane and Raynville Road, near Amen Corner.
It is understood builders working on a nearby housing estate may have accidentally ruptured the main.
Northern Gas Network operatives were on site and were understood to be waiting for the gas supply to be isolated so they could begin work on the leak.
No smoking signs were in place and it is understood road closures will come into effect almost immediately.