Videos posted online showed huge grey clouds racing towards climbers who then scrambled to safety as ash enveloped them.
“All of a sudden ash piled up so quickly that we couldn’t even open the door,” said Shuichi Mukai, who works in a mountain hut just below the peak which quickly became filled with hikers seeking refuge.
“We were really packed in here, maybe 150 people. There were some children crying, but most people were calm.”
An official working for Japan’s Meteorological Agency on volcano eruptions said there had been a rising number of small earthquakes detected at Ontake since 10 September, but that the eruption could not have been predicted easily.
Flights at Tokyo’s Haneda airport suffered delays as planes changed routes to avoid the peak, which lies 125 miles west of Tokyo, but were mostly back to normal by Sunday.
Japan is one of the world’s most seismically active countries and volcanoes erupt periodically, but there have been no deaths since 1991, when 43 people died in a superheated current of gas and rock at Mount Unzen in south western Japan.