The ceiling of the building that houses the No. 1 reactor at Fukushima No. 1 nuclear power plant collapsed Saturday, injuring 4 workers, Japan's public broadcaster NHK reported.

The accident occurred at around 3:36 p.m.(0636 GMT), a day after a 8.8 magnitude earthquake struck northeastern Japan, triggering massive tsunami on Japan's Pacific coast.

Authorities were still investigating to establish the cause of the accident.

Radioactive substances have been detected, and local authorities advised residents to stay clear of a 10-km radius of the nuclear plant.