Four people have been injured after an explosion rocked a busy district in Central Tokyo. It happened Monday afternoon near Shimbashi station.

The blast blew out the windows of a restaurant in a building on the west side of the station. The area is home to many offices and restaurants. The explosion sparked a fire and scattered pieces of glass and cardboard across the street.

Tokyo Metropolitan Police say restaurant staff were preparing to open. They say the manager told them the explosion happened when he lit a cigarette in a smoking room.

An expert says the blast may have been the result of a gas leak.

Tokyo University of Science professor Sekizawa Ai says: "The shutters of the first floor are not deformed and the glass windows of the floor above are not damaged. So my guess is that some kind of gas that accumulated in the space where there was fire, is to blame for the explosion."

The restaurant manager and one of his staff were among the injured. The two others who were hurt were passing by the building at the time of the explosion. Police are still investigating the cause.