Rescuers confirmed two fatalities after picking up some 80 people off the Bulgaria -- a 56-year old craft packed with about 150 tourists and more than 30 crew who were taking a traditional ride in a popular vacation spot.
But rescuers said the 47 divers who had reached the stricken craft by nightfall had found few signs of life.
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"The results of the inspection show that the chance of us finding survivors are minimal," an emergencies ministry official in Moscow told the Interfax news agency.
Survivors and other witnesses described a Sunday afternoon storm that suddenly made the two-deck boat tilt right in a wide bend of the river before it capsized and sank in a matter of minutes.
Officials had initially placed their hopes on a string of 13 islets nearby that could have provided potential shelter for those who slipped into the current.
But several weeping survivors draped in thick blankets described how they had been unable to save loved ones after climbing on board a nearby craft that had rushed to their rescue.
"My son-in-law telephoned to say that he held out his hand to his wife but she could not grab on," one man standing on a pier and staring into the water told Rossiya 24 state television. "He could not pull her out."
Another woman described her own child slipping out of her arms as the boat quickly filled with water as it sank.
"I held her to the last. She slipped out," the woman said.
"Almost none of the children survived. And there so many children -- far more of them than the adults. Some had two, three or four children with them," said the woman.
One man said around 30 children had gathered in a playroom on the second deck moments before the accident.
"I fear they all died," the unnamed survivor said.
"It flipped over in three minutes and sank. Lots of people died."
Emergencies Minister Sergei Shoigu discussed the disaster by telephone with President Dmitry Medvedev and then issued instructions for rescuers to continue searching through the night.
The transportation ministry later issued a statement saying the Bulgaria had passed a full safety check on June 15.
