An explosion at a building in Tokyo's Shimbashi commercial district on Monday left four people injured, Japan's state media NHK reported.

The explosion scattered debris across the streets, shattered windows and spewed smoke into the air.

The fire broke out in a building in a busy restaurant area around 3:20 a.m. local time (0620 UTC). Shimbashi is known as a business and entertainment district in central Tokyo.

Police rushed to the site after receiving an emergency call, local media reported.

What do we know about the blast in Tokyo?

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

He added that, "pieces of paper were scattered all over at the nearby intersection. From the way they were scattered, I immediately knew it was an explosion."

Footage on NHK showed flames in the shattered second floor windows of a building, while some witnesses said that they smelled gas before hearing an explosion. 

"A total of 32 fire trucks are at the scene," a fire department spokesperson said.

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

Authorities are investigating the cause of the fire.

ara/ab (AFP,AP, Reuters)