As many as 37 people were killed when a state transport bus in which they were travelling plunged from a bridge into Purna river in Navsari district of South Gujarat on Friday, police said.

"37 people have died in the incident, while 24 others are injured when a bus fell from a 20 feet high bridge on Purna river,' Navsari superintendent of police M S Bharada said.

Though the police is investigating the cause of the mishap, some survivors and eyewitnesses said the bus driver lost control of the vehicle when it was passing the bridge and it hurtled down into the river after breaking iron railings.

The SP said the condition of four out of the 24 injured passengers was serious.   "The injured have been shifted to four hospitals in Navsari. We are also checking the possibility of shifting those who are seriously injured to hospitals in Surat if needed," the officer said.

The Gujarat State Road Transport Corporation bus was headed towards Ukai when the accident took place near the village situated around 12 km away from the district headquarters.

"We do not know the exact number of people travelling in the bus, but it is said that it was full (to capacity)," Bharada said.

He said the agencies like police, fire brigade, 108 ambulance services were engaged in rescue operation at the spot.

Prime Minister Narendra Modi took to Twitter to condole loss of lives in the accident.

Bus accident in Gujarats Navsari is tragic and deeply upsetting. Condolences to families of the deceased. May the injured recover quickly, he tweeted.

Expressing grief over the incident, Chief Minister Anandiben Patel tweeted, 'Deeply pained to learn about the unfortunate bus accident on Purna River Bridge in Navsari. My prayers are with victims and families.'

'I have directed Navsari collector and concerned officials to expedite rescue and relief ops and provide required help to victims and their kin,' she tweeted.