Teams from army and disaster response forces had reached the area, local authorities said, with workers trying to rescue people trapped under debris and sludge, authorities added, Reuters reported.
TV news channels showed water, mud and debris surging down a mountain, sweeping away homes and a road.
The mudslide cleaved through Dharali village, burying some houses, according to a video update shared by the state chief minister's office.
"A massive mudslide struck Dharali village in the KheerGad area near Harsil, triggering a sudden flow of debris and water through the settlement," the Central Command of the Indian Army said in a post on X.
Uttarakhand is prone to floods and landslides, which some experts blame on climate change.
At least 200 people died in 2021 when flash floods swept away two hydroelectric projects in the state.
There are about 10,000 glaciers in the Indian Himalayas, and many are receding due to the warming climate.