The government of Nepal has declared a state of emergency after a 7.8-magnitude earthquake struck the country and killed hundreds of people, touching off a deadly avalanche on Mount Everest.

Officials said that more than 1,800 people were known to have died on Saturday in Nepal's most powerful quake in decades, that struck 80km east of Pokhara, about half way between the town and the capital Kathmandu.

As aftershocks continued throughout Saturday, the toll was was expected to rise significantly as the scale of the disaster became clear.

Of the dead retrieved so far, 634 were reported dead in the Kathmandu Valley and at least 300 more in the capital, a police spokesman told Reuters news agency.

A further 36 fatalities were reported in northern India, 12 in Tibet and four in Bangladesh.

Photos posted on Twitter showed buildings left in rubble, large cracks along roads and worried residents on the streets.

Images of devastation: Nepal's earthquake destruction

The earthquake destroyed many historical landmarks, including the UNESCO World Heritage temples at Basantapur Durbar Square and the Dharara tower, both in central Kathmandu.

It has been reported that around 250 people may have been in the Dharara tower when it collapsed. At least 50 people are believed to be trapped under an ancient tower in Kathmandu which collapsed in the quake.

Emergency workers and army and police personnel, with the help of residents and bystanders, continued to work tirelessly on Saturday to clear the rubble from these sites and to rescue any survivors from under the debris, although bodies were mostly being pulled out.

As night fell the country, thousands of people were staying outdoors and found refuge in Kathmandu’s open spaces, in fear that subsequent aftershocks may cause further damage.

The epicentre was 80km northwest of Kathmandu. The Kathmandu valley is densely populated with nearly 2.5 million people and poorly enforced building regulations.

The Associated Press news agency cited a senior guide as saying that an avalanche swept a mountain near the Everest base camp. Al Jazeera has learned that at least 10 people were killed in the incident, which also left many climbers trapped.

British mountaineer Kenton Cool told Al Jazeera that the quake couldn't have come at a worse time, as it was peak climbing season at the base camp and "there's probably 1,000 people or so there". Cool said conditions for rescuers would also be difficult in poor weather conditions.

"This terrible, terrible situation has occurred at exactly the wrong time in terms of human fatality," Cool said.

"Knowing the geology and the lay of the land there, to facilitate the medical care required and to try to medivac people out of there is going to be extremely difficult, especially considering the weather conditions there right now."

Al Jazeera's Subina Shrestha, reporting from Kathmandu, confirmed reports of extensive damage to buildings.

The worst-affected area was old Kathmandu, the site of several ancient temples, and where rescuers faced large piles of rubble, she said.

As night fell in the country, tens of thousands of people camped out, fearful of going back to their homes in case of another earthquake. Already, more than two dozen aftershocks had followed the quake.

The US Geological Survey said the quake hit at 11:56am local time (06:11 GMT). It initially reported a magnitude of 7.7 before revising its calculation to 7.5, 7.9 and then later, 7.8.

The USGS initally reported the epicentre to be 11km deep but later revised it to 2km.

Tremors were felt in a number of India's northern cities, witnesses said. Al Jazeera's reporters in New Delhi said the tremors were also felt across the Indian capital.

Al Jazeera's Faiz Jamil said that the first tremor in New Delhi lasted for 20 seconds and a second one followed shortly after.

Utter devastation. Locals pitch in to clear rubble, search for survivors. #kathmanduearthquake pic.twitter.com/Td9KeGGfmX — Siobhan Heanue (@siobhanheanue) April 25, 2015

Indian officials told the Associated Press news agency that at least 34 people were killed in the Bihar state and Uttar Pradesh state.

Al Jazeera's Maher Sattar in Dhaka, Bangladesh, reported that at least three people were killed there, including one who was killed following stampede triggered by the quake.

Laxman Singh Rathore, director-general of the Indian Meteorological Department, said that the impact had been felt across large swathes of northern India.

"The intensity was felt in entire north India. More intense shocks were felt in eastern UP (Uttar Pradesh) and Bihar, equally strong in sub-Himalayan West Bengal, Sikkim," he said.

(Additional reporting by Juliette Rousselot in Kathmandu)