Thousands were also evacuated from near a second plant, Fukushima No. 2, which also suffered damage to its cooling system.
The earthquake unleashed a terrifying 10-meter (33-foot) wave that tore through coastal towns and cities, destroying all in its path and leaving thousands of people missing.
Smoke billowed from the Fukushima No. 1 atomic plant, about 250 kilometres (160 miles) northeast of Tokyo, after the explosion at the aging facility destroyed the walls and roof, reports said. The blast reportedly left several workers with non-life-threatening injuries.
The atomic emergency came as the country struggled to assess the full extent of the devastation wreaked by the massive tsunami, which was unleashed by the strongest quake ever recorded in Japan.
The towering wall of water pulverised the northeastern city of Sendai, where police reportedly said 200-300 bodies had been found on the coast.
More than 215,000 people were in emergency shelters, police said.
The full scale of those left homeless was believed to be much higher, with police saying they had not received a tally from Miyagi prefecture, the hard-hit province that is home to Sendai.