Tens of thousands of people, including pilgrims, tourists and local villagers, remained stranded on Thursday and an unknown number have been killed after torrential rains in the eastern Himalayas breached a glacier, flooded mountain rivers and triggered scores of landslides.

The horrific natural disaster, described by some as a "Himalayan tsunami", was triggered by excessively heavy rainfall of more than 220mm (8.6in) on Sunday in a region home to the headwaters of the river Ganges.

As army helicopters ferried survivors down to the plains for a second day on Thursday, the official death toll passed 150, but unofficial estimates put the number of dead in the thousands.

Floods and landslides are an annual occurrence in Uttarakhand state – where the disaster struck – but the loss of life is much larger than in previous years as the monsoon arrived in India a month early, well before more than 60,000 Hindu and Sikh pilgrims who had trekked to the Himalayan holy sites of Kedarnath and Hemkund had returned home.

According to the Hindu religious calendar, Tuesday marked the day on which the Ganges descended to earth and when the rush of pilgrims is at its peak at the ancient Shiva temple in Kedarnath, a steep nine mile (14km) trek from the nearest bus stop.

Just three miles further north across the Kedar Dome mountain peak lies the Charbari glacier. The rains appear to have breached this glacier, sending a huge torrent of ice, rock, mud and water across the mountain, engulfing Kedarnath town.

It is unclear how many pilgrims and priests died at Kedarnath, but the account of one survivor, Kalyan Singh Jadaun, a middle-aged shopkeeper from Rajasthan, suggests a heavy loss of life.

Jadaun and his wife had just emerged from a guest house at 6.30am on Monday to join a stream of pilgrims heading toward the temple when the debris began rushing in.

"We were walking the 50 metres to the temple when we saw a huge chunk of ice and rock breaking not far away, and heard commotion and shouting," he told the Indian Express.

"Local priests were telling people to run to high ground as the glacier broke up and a huge stream came rushing down the mountain. We ran back to our house. There was a mad scramble to climb the stairs to the third floor as everyone tried to reach as high as they could."

In the melee, his wife was pushed back and when the torrent of water hit the building she was still on the second floor. "It swept her away, tossing her along with rocks and huge blocks of ice," said Jadaun. "I saw her being washed away."

Jadaun was rescued by an army helicopter. He said that after the deluge he saw "a large number of bodies strewn around" Kedarnath.

Heavy rain and swollen rivers also caused havoc over a swath of mountainous territory extending from Kedarnath down to the plains, washing away homes, hotels, roads and bridges. The impact was also felt in the western Himalayan state of Himachal Pradesh, where many foreign tourists were also stranded. Even parts of the capital, Delhi, are flooded due to the overflowing river Yamuna.

"Several factors, both natural and man-made, came together to cause this extraordinary disaster," said Deb Mukherjee, an expert on the Himalayas. "The sudden cloud burst, the nature of Himalayan geology and the often environmentally unsound development of the region have all contributed to the tragedy."