The bus, en route to Kathmandu from Rajbiraj, fell into the Trishuli River while negotiating a turn at Ghatabesi, 70 km west of Kathmandu, around 5:00 am, said Superintendent of Police, Dhading, Dhruba Raj Raut.
Security personnel, including those from the Nepal Army and Armed Police Force, rescued 16 injured passengers who had fallen into the river in the accident along the Prithvi Highway.
The rescue teams have pulled out 19 dead bodies so far and the search was on for other missing passengers, Raut said.
According to preliminary reports, the accident occurred because the driver failed to take the high speed bus under control at the turning point due to poorly visibility, the police said.
Altogether 52 passengers were travelling on the bus, police said.
While the police prima facie blamed visibility for today's accident, such mishaps are not uncommon in the Himalayan nation and occur mostly due to poor infrastructure and reckless driving.