Prime Minister Narendra Modi, who visited Bhopal during the day, mourned the mishap.
So far 14 bodies have been pulled out from the bus, Ratlam district collector Chandrashekar Borkar said, adding three persons were rescued.
The bus was on its way to Jaora from Ratlam when the mishap occurred on the four-lane road at 12 Patthar Khadan, about two km from Namli town, he said.
The bus fell into a pit dug for the soil/gravel to be used in road-laying. Rain water had collected in the pit.
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A National Disaster Response Force team from Bhopal and divers from Indore were called to carry out rescue operation.
The Prime Minister expressed grief over the incident.
"Extremely distressed by the loss of lives caused by the bus accident in Ratlam, MP. My thoughts are with the bereaved families.
"I wish those injured in the bus accident in Ratlam a quick recovery," Modi said in two tweets.
Unconfirmed reports said there were 40-45 passengers in the bus. According to eyewitnesses, it was going at a high speed when it overturned and fell into the pit.
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