Mount Ontakesan in central Japan erupted Sept. 27 for the first time since 2007, sending climbers scrambling for safety and injuring at least 27 people.
Ten of the 27 were seriously injured or rendered unconscious, Kiso fire authorities in Nagano Prefecture said.
Hundreds of climbers were believed to be on the mountain, having ventured out on a clear, sunny morning. As night fell, many continued to make their descent, arriving at the base of the mountain covered in ash.
The volcano, which straddles the borders of Nagano and Gifu prefectures, belched to life at 11:53 a.m. with a towering column of smoke and ash, the Japan Meteorological Agency said.
The activity prompted the agency to raise its volcano warning to level 3, which restricts entry to the 3,067-meter peak, a popular climbing spot and object of worship.
Prime Minister Shinzo Abe instructed the Self-Defense Forces personnel to be sent to the site.
Nagano prefectural officials said hikers reported to Kiso fire authorities that many climbers had collapsed on the trekking trail.
At one point, more than 250 climbers were stranded near the peak.
According to the National Police Agency, 230 had descended by 8:50 p.m., but 44 people remained in huts for climbers, including workers at the facilities.
Cameras installed by the land ministry's Chubu Regional Development Bureau showed volcanic plumes sweeping more than 3 kilometers down the mountain's southern slope.
The meteorological agency also issued a warning for flying debris in a 4-kilometer radius of the crater.
Yukio Hayakawa, a professor of volcanic geology at Gunma University, said there are indications that pyroclastic flows are getting near to the sprawling foot of the mountain, adding to concerns about climbers.
The meteorological agency said an eruption of similar scale could occur, although a significantly large one is not expected.
The agency added that it is difficult to tell when the volcanic activity will cease.