The injured were in mountain lodges, because they were unable to descend 3,067-meter Mount Ontake on their own, said Sohei Hanamura, a crisis management official in Nagano prefecture. Thirty-two people had serious injuries, including at least seven who lost consciousness.
Police, fire and military rescue workers were trying to approach the area on foot, after deciding that the ash in the air made it too dangerous to use helicopters. The ash was also hampering their ascent.
Lodge managers were familiar with first aid procedures and were communicating with rescue officials in town, he said.
With a sound likened to thunder, the volcano erupted shortly before noon on a clear autumn day, spewing large white plumes of ash high into the sky and sending people on the mountainside fleeing.
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The eruption continued into the night, blanketing the surrounding area in ash. About 250 people were initially trapped on the slopes, but most had made their way down by tonight, Japanese public broadcaster NHK reported. Some were in shelters set up in four nearby towns.
In a YouTube video, shocked climbers can be seen moving quickly away from the peak as an expanding plume of ash emerges above and then engulfs them.
Many of those who made it down emerged with clothes and backpacks covered in ash. They reported being engulfed in total darkness for several minutes.
Mikio Oguro, an NHK journalist who was on the slope on an unrelated assignment, told the station that he saw massive smoke coming out of the crater, blocking sunlight and reducing visibility to zero.
"My colleagues later told me that they thought they might die," Oguro said.
Two Jetstar flights headed to Tokyo's Narita International Airport diverted to Kansai International Airport in western Japan as a precaution.
Japan's meteorological agency raised the alert level for Mount Ontake to 3 on a scale of 1 to 5. It warned people to stay away from the mountain, saying ash and other debris could fall up to 4 kilometres away.