At least one woman died in the eruption, the broadcaster NHK reported on Saturday night, citing local fire officials.
More than 70 people were receiving medical treatment, 32 of them seriously injured and 10 of those unresponsive, as people coated with volcanic ash continued to arrive near the base of 3,067-metre Mount Ontake in central Japan.
"I first thought it was thunder as I heard a bang and another bang, two or three times," a trekker told NHK. "Then volcanic dust fell noisily."
Amateur cameraman Keiji Aoki told Jiji Press: "It was tremendous. I prepared for death when I got caught in the dust under a pine tree.