Television footage showed that large numbers of offices and homes had collapsed in Kathmandu while roads had been split in two by the force of the impact.
Panicked residents rushed into the streets as the tremor erupted around midday, with the impact felt hundreds of miles away in big swathes of northern India and even in Bangladesh.
The United States Geological Survey said the quake struck 81 kilometres northwest of Kathmandu at 1141 IST, with walls crumbling and families racing outside of their homes.
Another resident recounted scenes of panic and mayhem.
More From This Section
"Everything started shaking. Everything fell down. The walls around the main road have collapsed. The national stadiums gates have collapsed," Kathmandu resident Anupa Shrestha said.
While there were no immediate reports of deaths, police confirmed that many buildings had collapsed.
Witnesses said Kathmandu's landmark Dharahara tower also collapsed due to the quake.
"Our focus is on rescue in the core areas of Kathmandu where the population is concentrated," Dinesh Acharya, metropolitan police spokesman, said.
Initially measured at 7.5 magnitude, the quake was later adjusted to 7.9, with a depth of 15 kilometres, the USGS said. It hit 68 kilometres east of the tourist town of Pokhara.
Witnesses and media reports said the quake tremors lasted between 30 seconds and two minutes.