More than 30 hikers were feared dead Sunday, a day after a volcano in Japan erupted without warning, spewing ash and rocks down slopes crowded with people on a sunny autumn weekend.

Rescue efforts were still underway Sunday night on Mount Ontake, a 10,000-foot-high volcano about 125 miles west of Tokyo, as the eruption continued.

But 31 people had been discovered at the top of the mountain in a state of “cardiopulmonary arrest,” local media reported, using the terms for heart and lung failure that Japanese authorities employ to describe bodies. People cannot be declared dead in Japan until doctors have examined them.

“We have confirmed that more than 30 individuals in cardiac arrest have been found near the summit,” a police spokesman in Nagano prefecture told Agence France-Presse.

Four of the victims were later brought down and confirmed dead, according to a crisis-management official cited by the Associated Press. Police had identified two of them, the AP said — men ages 23 and 45.

Some of the bodies were found in a lodge near the summit and others were buried in ash up to 20 inches deep, according to Japanese media.

More than 500 police, firefighters and military personnel were involved in a rescue operation on the mountain Sunday, bringing down some climbers on stretchers and others in helicopters.

Television footage showed other climbers, in brightly colored hiking gear and yellow hard hats, trekking through what looked like a moonscape — a gray mountain covered in volcanic ash and rocks and shrouded in clouds.

With its well-marked trails and numerous lodges, Mount Ontake is a popular hiking destination, especially during September and October, when the leaves are turning red. But the volcano erupted without warning shortly before lunchtime on Saturday.

Shinichi Shimohara, who works at a shrine at the base of the mountain, said he was starting to climb when he heard what sounded like strong winds followed by what he thought was thunder.

“For a while I heard thunder pounding a number of times,” he told the AP. “Soon after, some climbers started descending. They were all covered with ash, completely white. I thought to myself, this must be really serious.”

Some climbers who were already up on the mountain crammed into buildings.

Firefighters and members of the Japan Self-Defense Forces carry a person rescued near the peak of Mount Ontake in central Japan Sunday. (Kyodo News/Associated Press)

“All of a sudden, ash piled up so quickly that we couldn’t even open the door,” Shuichi Mukai, who worked in a mountain lodge just below the peak, told Reuters. “We were really packed in, maybe 150 people. There were some children crying, but most people were calm.”

Others reported hiding underneath buildings when the roofs caved in under falling rocks.

But the rescue operation was hampered by ongoing seismic activity on the mountain. Recovery efforts were suspended for a period on Sunday, and rescue workers were forced to descend because of the strong smell of sulfur, NHK reported.

Japan’s Meteorological Agency said that the eruption was continuing Sunday evening.