At least 10 people were killed and several others went missing after a motorboat carrying around 115 passengers capsized on Sunday in the Hirakud Dam reservoir in Odisha's Sambalpur district.
Police said more than 80 people had been rescued while a search was on for another 25 people.
Odisha special relief commissioner PK Mohapatra said the death toll was likely to rise.
"The Odisha Disaster Rapid Action Force personnel and fire brigade have launched a massive rescue operation. The final death toll will be known after the operation is over," Mohapatra said.
Odisha chief minister Naveen Patnaik condoled the deaths and asked the Sambalpur district administration to speed up the rescue operation.
The boat was carrying mostly picnickers, including women and children. It developed a snag in its motor pump and was flooded, officials said.
Eyewitnesses said around 100 members of the Lion's Club of Sambalpur had gone into the reservoir in two boats for a picnic. However, one of the boats became overcrowded while returning as several regular passengers also came on board.
Hundreds of villagers regularly commute by motorboats between Sambalpur, Bargarh and Jharsuguda districts that surround the reservoir.?