Footage from the scene showed smoke pouring out of the windows of the fourth floor, where the blaze broke out, as well as the roof of the multi-storey office building.

A total of 70 fire engines were sent to fight the flames, which were mostly extinguished within about 30 minutes of the emergency call, the authorities said.

Later images showed the windows, blackened and charred.

Some 27 people were in cardiopulmonary arrest, an official at Osaka's fire department said - a term used in Japan before a death is officially confirmed.

Another person was injured.

Police are investigating suspected arson with reports that an elderly man, believed to be patient, had set fire to a bag of flammable liquid, shortly after the clinic opened.

A 36-year-old woman who works at a nearby company told local media: "When I looked outside I saw orange flames in the fourth-floor window of the building.

"A woman was waving her hands for help from the sixth floor window."

The clinic's webpage had previously showed it treated patients for depression, panic attacks and sleeping problems.

Located in a shopping and entertainment district not far from Osaka's main train station, the building also houses a beauty salon, a clothes shop and an English-language school, according to reports.