TOKYO — At least 41 people were injured in an explosion on Sunday night at a restaurant in northern Japan, the police said.

Television footage from the Japanese public broadcaster NHK showed the restaurant in flames, with debris scattered on the ground in Sapporo, the capital of the northern island of Hokkaido.

Most of those hurt had mild injuries, but one person was in serious condition.

The cause of the explosion, which occurred at 8.30 p.m. local time, is under investigation, the police said. They offered no further details.

A witness told NHK that he had smelled gas after the sound of an explosion.

The video showed gray smoke billowing from the restaurant as dozens of firefighters poured water onto the building, which was charred and nearly collapsing.