TOKYO—An explosion tore through a building in downtown Tokyo on Monday, scattering debris across a busy intersection and sending smoke into the air, but the fire was soon contained, Japanese media said.

Four people were injured, three seriously, but all were conscious, said public broadcaster NHK, which aired video images of flames through the windows of the shattered second floor of the building in the Shinbashi area of the Japanese capital.

The cause of the blast was not immediately clear but witnesses said they had smelled gas before the explosion.

“I’d just got to work and was starting preparations when there was a really loud explosion,” said Shinobu Nakagawa, a 26-year-old restaurant worker, who works on the first floor of the building. He said he grabbed his phone and rushed outside.

“Furniture and shards of glass were scattered everywhere, and when I looked up, smoke was surging upwards.”

The blast took place in an area near a railway station that is packed with bars and restaurants popular with office workers.