At least 31 hikers have been found lifeless near the peak of Mount Ontake after Saturday’s volcanic eruption.
The hikers were not breathing and their hearts had stopped, reports said, but there will be no confirmation of death until after a medical assessment.
About 250 people were trapped on the slopes of the popular beauty spot, but most have got down safely.
The volcano, about 200km (125 miles) west of Tokyo, erupted without warning on Saturday, spewing ash and rocks.
The eruption forced many of those on the mountain to make emergency descents through clouds of volcanic ash and falling rocks.
“The volcanic rocks fell like hailstones,” one man said.
“We couldn’t breathe so we covered our mouths with towels. We couldn’t open our eyes either.”
Another told reporters: “The volcanic ash was hurtling so fast I couldn’t run away. I’m worried about people still on the mountain.”
Almost 50 people were thought to have stayed on the mountain overnight, reports said.
As the search effort intensified earlier on Sunday, officials said they were searching for 30 people feared missing or buried by ash.
Confirmation soon came that a similar number of people had been found unresponsive on the mountain.
“We have confirmed that more than 30 individuals in cardiac arrest have been found near the summit,” a Nagano police spokesman told the AFP news agency.
Japan’s NHK News put the figure at 31, with four said to be transported back down the mountain on Sunday.
Military helicopters plucked seven people off the mountainside earlier on Sunday, according to reports, and workers on foot were also helping others make their way down.