Leicestershire Police reported only that a "major incident" had occurred and multiple streets had been closed.
Traffic patterns on and around Hinkley Road have now been suspended as emergency responders fight the fire.
Leicestershire Police have confirmed on their website that six people had been taken to hospital "and we await an update on their condition".
Officers have said they are conducting a search and rescue operation at the scene.
On Twitter, Mark Robins wrote: "Jesus, huge explosion Hinckley road area about 10 minutes ago". A "major incident" has been declared by police. The old Londis and the flat above have been destroyed by fire.
Another neighbor said "there was an terrible lot of smoke", Sky News reported.
It is not yet clear whether anyone was inside the building, which appears to have been blown to pieces, at the time of the explosion.
The cause of the blast remains unknown, although a hazmat team is reportedly on scene investigating.
There were no immediate indications that the explosion was linked to terrorism, broadcaster Sky News reported, citing unidentified sources. We could see the flames.
The incident left four people critically injured.
A witness at the scene told CBS News she heard a "bang" and thought it was a vehicle accident. One man said he thought it sounded like a bomb.
Residents living close to Hinkley Road, about a mile from the city centre, said their houses shook with the force of the blast.