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."
A suffocating blanket of ash up to 20 centimetres deep covered a large area of the volcano, trapping climbers and forcing up to 150 into mountaintop shelters at one point.
About 230 had made it to the bottom of the volcanic peak, where eruptions of ash, rocks and smoke continued, as of 8.30pm local time, NHK said.
"There appear to be many injured," Chief Cabinet Secretary Yoshihide Suga said in a televised press conference earlier in the evening. "The government is still confirming how many people are trapped on the mountain."
At least seven people were buried in ash from the eruption, only one of whom has been found and was unresponsive, according to a report on the Cabinet office website. Among the injured were five with broken bones, while the condition of others was undetermined, the report indicated.
About 200 people took shelter at a lodge near the summit of the volcano after the eruption, the first in seven years, NHK said earlier, citing telephone contact with those at the lodge.
Conditions were too dangerous for anyone to leave the lodge to try to assist people who could be seen collapsed along trails on the mountain, according to that report.