Four people were injured in a blast and a blaze in a central Tokyo building, the city's fire department said, with footage showing flames and smoke emerging from a second-floor business.

The fire started in a building in a restaurant area of the bustling Shimbashi district around 3:20 pm.

"The sound of an explosion was also heard. A total of 32 fire trucks are at the scene," a fire department spokesman told AFP, declining to be named.

"Four people are injured, three of whom are conscious," he said, adding no details were yet available on the fourth person.

Officials are investigating the cause of the fire.

"The sound was deafening. It was as if something huge was dropped from the top of the building -- it was an incredible bang," a male eyewitness told public broadcaster NHK.

"Pieces of paper were scattered all over at the nearby intersection. From the way they were scattered, I immediately knew it was an explosion," he said.