There was no indication that the explosion was linked to terrorism.
“The cause of the explosion will be the subject of a joint investigation by the police and Leicestershire Fire and Rescue Service,” local police said in a statement.
“We would ask that the media and public do not speculate about the circumstances surrounding the incident, but at this stage there is no indication this is terrorist related.”
The police said that six fire crews were on scene, including a specialist search and rescue team, supported by two search dogs.
“We will work closely with colleagues from the police as our enquiries into the circumstances continue today,” the police said.
The apparent explosion occurred Sunday evening on a stretch of road containing commercial and residential properties close to the city center.
The electricity supply to a number of properties in the area was cut while emergency services deal with the incident, police said.
The Fire and Rescue Service said it received calls shortly after 7:00 p.m. (1900 GMT) from the public reporting an explosion and a building fire.
It immediately dispatched six fire engines, a spokeswoman said.
“We were en route when the police called to say a building had collapsed,” she added.
Additional specialist search and rescue units were then dispatched, including a search and rescue dog, according to the spokeswoman.
“We’ve got no indication of what the cause is at this moment,” the spokeswoman said.
Six people were taken by ambulance to Leicester Royal Infirmary hospital following the incident, according to University Hospitals of Leicester, which runs three area medical facilities.
“Two of these patients are in a critical condition, while four are walking wounded,” it said on Twitter.
Officials had earlier advised people only to attend the accident and emergency department “if absolutely necessary.”
The fire service spokeswoman also said representatives from gas and electric provider companies were in attendance at the scene, under routine procedures.
Pictures and videos posted on social media showed a property engulfed in flames visible from a distance, with rubble and debris scattered around.
Graeme Hudson told AFP he lives close to the scene and felt the blast.
“It was very scary,” he said. “I live five minutes away... but my house shook. I went out and saw massive smoke and big flames.
“I quickly went out to see what’s happened,” he added. “(I) didn’t stay for long there because (I felt) unsafe for my 11-year-old son.”