At least 250 hikers, including children, were visiting the popular spot a little over 200 kilometres west of Tokyo. It was a clear day and the peak offered a relatively easy climb amid colourful fall foliage.
The noise of the eruption was the first sign anything was wrong.
"It was like thunder," a woman told public broadcaster NHK of the eruption, which killed at least 36 and injured dozens more. "I heard boom, boom, then everything went dark."
A thick, rolling, grey cloud of ash rose into the sky above the mountain.
"I felt a hot wind blast against my back and crouched down to the ground," a man told the Nippon Television Network. "I was sure I was going to die."
Shocked climbers scrambled for safety as the expanding plume of ash emerged above and then engulfed them, leaving many survivors in total darkness for several minutes.
"Massive ash suddenly fell and the entire area was totally covered in ash," reported Mikio Oguro, a journalist with NHK. Oguro told his network that his crew had to use headlamps to find a lodge.
'I couldn't stop him'
Yuji Tsuno, a veteran mountain photographer, was near the summit. After taking pictures of the initial explosion as ash and debris rained down, he quickly took refuge in a nearby hut, he told the TBS TV network.
About 20 minutes later, when the smoke partially subsided, he rushed out and began his descent. It was a gamble, but he believed it was his only chance, he said.
"I almost thought it was the end of my life," he said.
On his way down, he spotted a man heading up. "I told him to go down with me, but he said he had to check on his child up there. I couldn't stop him," Tsuno said.
Survivors also told of being pelted by falling rocks.
"The volcanic rocks fell like hailstones," one man said, according to the BBC. "We couldn't breathe so we covered our mouths with towels. We couldn't open our eyes either."
One woman said she covered her head with a knapsack, and later found a Thermos inside had been flattened. One man said he and others went into the basement of a lodge, fearing that the rocks would penetrate the roof. He covered himself with a thin mattress for protection.
NHK footage showed windows in one mountain lodge darkening and people screaming as heavy objects pelted the roof.
"There was billowing heat everywhere as huge falling rocks pierced the ceilings in the shacks we were taking refuge in," one man said, according to English-language Chinese broadcaster CCTV. "I thought I would die for sure — almost gave up on trying."
Shuichi Mukai, who worked in a mountain lodge just below the peak, told Reuters that ash piled up so quickly that for a time they couldn't open the door. The building quickly filled with hikers.
"We were really packed in, maybe 150 people. There were some children crying, but most people were calm. We waited there in hard hats until they told us it was safe to come down."
ShinichiShimohara, who works at a shrine at the foot of the mountain, said he was on his way up Saturday morning when he heard what sounded like strong winds followed by thunder.
"For a while I heard thunder pounding a number of times," he said. "Soon after, some climbers started descending. They were all covered with ash, completely white. I thought to myself, 'This must be really serious.'"