The driver of the truck that was coming from Coimbatore is on the run, the police said.
Over 19 people were killed after the bus in which they were travelling in was hit by a lorry coming from the opposite side on a highway in Tamil Nadu's Tirupur, the police said. Twenty-three were injured. They have been taken to different hospitals.
The Kerala government-run Volvo bus that can seat over 50 people was going to Ernakulam from Bengaluru in Karnataka, state Transport Minister AK Saseendran told reporters.
The police said the number of dead is likely to rise.
In visuals, the right side of the white bus is seen completely mangled, the driver's area is smashed in and the damaged bus lies tilted.
Reports said the tyre of the truck burst while it was in speed, and it rolled on to the road before colliding with the bus.
Rescuers at the accident site were trying to find survivors in the debris.
The driver of the truck that was coming from Coimbatore is on the run, the police said.
"All possible relief measures will be taken in cooperation with the government of Tamil Nadu and the District Collector of Tirupur," Chief Minister Pinarayi Vijayan's office said.
The driver and the conductor, VD Girish and VR Baiju, had just a year got an appreciation letter from the government for rushing a Palakkad resident to a hospital in Hosur. She had fainted and fell down in the bus and they stayed with her till her relatives arrived.
"Extremely anguished by the bus accident in Tamil Nadu's Tiruppur district. In this hour of grief, my thoughts and prayers are with the bereaved families. I hope those who are injured recover at the earliest," Prime Minister Narendra Modi tweeted.