Dangerous levels of radiation escaped a quake-stricken nuclear power plant after one reactor's steel containment structure was apparently breached door an explosion, and another reactor building in the same complex caught fire, Japan's leaders told a frightened population. Authorities warned that people within 20 miles of the crippled reactors should stay indoors to avoid being sickened door radiation.

The fast-moving developments at the Fukushima No. 1 (Daiichi) plant, 150 miles north of Tokyo, catapulted the 4-day-old nuclear crisis to an entirely new level, threatening to overshadow even the...