A passenger train derailed on flooded tracks in southern India on Saturday and plunged into a rain-swollen river, killing at least 100 people and trapping scores inside submerged cars, officials said.

About 100 injured passengers were rescued from the train, which derailed after floods washed away the tracks in the town of Veligonda in Andhra Pradesh state.

"We were fast asleep, when there was a big bang and a thud. The next thing the train was under water," said P. Ramesh, a passenger who lost seven members of his family in the wreck, including his wife and brother.

Army divers and local volunteers swam to the coaches to help pull out the injured. Other soldiers, lowered onto the roofs of the coaches by helicopter, cut through the top of rail cars and retrieved passengers hanging on to luggage racks and ceiling fans.

Many people were still trapped inside the cars, at least five of which were lying on their sides, partially submerged. One of the cars was resting on top of another.

"I was able to clamber out of the coach, but others were not so lucky," said Ramesh, the rescued passenger.

The train -- an engine and 17 cars -- hit a portion of track washed away by flash floods, and seven coaches derailed, officials said.

Rains also washed away many roads in the area, making it difficult for rescuers and ambulances to reach the accident site. Traffic jams stretched for miles on roads leading to Veligonda.

India's 39,233-mile rail network is the world's third-largest after Russia's and China's. Every day, at least 14 million people use India's state-run network, which has poor safety standards and is plagued by accidents. Around 300 accidents involving trains occur in India each year.