AT least 31 people have been found presumed dead near the peak of a Japanese volcano that erupted unexpectedly while it was packed with hikers out to admire autumn foliage, sending a huge cloud of ash and rock tumbling down its slopes.

Police said the 31 people were found in cardiopulmonary arrest but declined to confirm their deaths pending a formal examination, as per Japanese custom.

An official in the area said rescue efforts had been called off due to rising levels of toxic gas near the peak of Mount Ontake, as well as approaching nightfall.

Hundreds of people, including children, were stranded on the mountain, a popular hiking site, after it erupted without warning on Saturday, sending ash pouring down the slope for more than two miles.

Most made their way down later on Saturday but about 40 spent the night near the 10,062 feet peak. Some wrapped themselves in ­blankets and huddled in the ­basement of buildings.

More than 40 people were injured, several with broken bones.

The volcano was still erupting yesterday, pouring smoke and ash hundreds of feet into the sky. Ash was found on cars as far as 50 miles away.

Volcanoes erupt periodically in Japan, one of the world's most seismically active nations, but there have been no fatalities since 1991, when 43 people died in a pyroclastic flow, a superheated current of gas and rock, at Mount Unzen in southwestern Japan.

Ontake, Japan's second-highest volcano 125 miles west of Tokyo, last erupted seven years ago.

Its last major eruption was in 1979.

Satoshi Saito, a 52-year-old hiker who climbed Ontake on Saturday and descended less than an hour before the eruption, said the weather was good and the ­mountain, known for its autumn foliage, was crowded with people carrying cameras.

He said: "There were no earthquakes or strange smells on the mountain when I was there. But a man who runs a hotel near the mountain told me the number of small earthquakes had risen these past two months, and everyone thought it was weird."

Images online showed huge grey clouds boiling towards ­climbers at the peak and people scrambling to descend as blackness enveloped them.

Shuichi Mukai, who worked in a mountain lodge just below the peak, said: "All of a sudden ash piled up so quickly we couldn't even open the door. 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."

Flights at Tokyo's Haneda airport suffered delays as planes changed routes to avoid the volcano, which straddles Nagano and Gifu prefectures.

But things were mostly back to normal by yesterday, an airport spokeswoman said. An official at the volcano division of the Japan Meteorological Agency said that, while there had been a rising number of small earthquakes detected at Ontake since September 10, the eruption could not have been predicted easily.

The official said: "There were no other signs of an imminent eruption, such as earth movements or changes on the mountain's surface.

"With only the earthquakes, we couldn't really say this would lead to an eruption." Japan lies on the Ring of Fire, a horseshoe-shaped band of fault lines and volcanoes circling the edges of the Pacific Ocean, and is home to 110 active volcanoes.

One of these, Sakurajima at the southern end of the western island of Kyushu, is 31 miles from Kyushu Electric Power's Sendai nuclear plant, which was approved to restart by Japan's nuclear regulator earlier in September.