Emergency personnel at the scene on Hinckley Road in Leicester, after a "major incident" was declared by police after reports of an explosion at around 7.03pm, from which four people have been taken to hospital, East Midlands Ambulance Service said.
They also said people may be still be missing following the incident.
"We've now got specialist search and rescue teams supported by search dogs on scene", Matt Cane, from Leicestershire Fire and Rescue, told AFP at the site.
"Loads of people were gathered and there was gas and flames coming out of the building", Khan said.
The cause of the reported explosion has not been determined.
Leicester is 180 kilometres north of London. "It is unknown if people are injured or trapped".
"I heard a big bang, I thought, 'What was that?' and found myself on the floor ... my body was half covered in rubble ..." "The building consisted of a shop premises on the ground level and a two-story flat above it".
One witness told the Mirror: "It's blown the shop across both sides of lanes and there is carnage everywhere".
"Forget about the money, the most important [thing] is life", he said.
"He is in a daze, he said his mum and his brothers were watching TV and then the next thing he knows bricks were falling over him, and then it was pitch black".
Therefore, the fire service is now unable to determine the cause, which may take a number of days to become clear, he said.
The apparent explosion occurred Sunday evening on a stretch of road containing commercial and residential properties close to the city center.
"Carlisle Street and part of Hinckley Road have been closed".
Julie Dexter, 56, told The Sun online: "It felt like the house was falling down".
"The cause of the explosion will be the subject of a joint investigation by the police and Leicestershire Fire and Rescue Service", police said.