More than a dozen people died in the state's Rudraprayag district alone, said Amit Negi, an official in Uttarakhand.
A landslide triggered by the monsoon rains buried a bus, killing three people in Almora district.
At least three other people were washed away when a three-story apartment building toppled into a river and was carried away by the swift-moving current, said Amit Chandola, a government spokesman.
More than 10,000 pilgrims stranded along a mountain pass leading to a Hindu religious site were being evacuated by helicopter.
Army troops were leading efforts to rescue scores of people from the rooftops of their flooded homes.
The River Ganges and its tributaries are flowing above the danger mark in several areas in the Himalayan state.
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