Four victims have been taken to Frankford Torresdale Hospital. Three are PGW workers and one is a Philadelphia firefighter.
There are unconfirmed reports of one missing person.
The incident was first reported as a hazmat scene around 7:30 p.m., when a 12-inch high-pressure gas main ruptured.
Numerous repair workers responded to the call.
The explosion was reported about one hour later as crews were trying to repair the gas-main break. There may have been a PGW truck near the explosion scene.