Eyewitnesses said that the incident took place around 7 pm.
The truck carrying the pilgrims turned turtle and lunged down the 30-feet deep gorge en-route to the Sabarimala temple, dedicated to Lord Ayyappa.
The injured were rushed to the Kottayam Medical College after being administered first aid.
Talking to a News agency, K Sivadasan Nair, the local legislator said that the truck carried over 40 people and many sand-filled plastic bags had been placed in the vehicle to ensure a smooth ride.
"This is really sad...how can people and that too so many of them travel on a lorry? How can this happen? There should be a clear guideline on transportation. And authorities have to take care of the movement of pilgrims," said Nair.
He added that the many locals came to the rescue of those injured.
"Local people engaged in the rescue operations said all those involved in the accident have been rescued. The locals rose to the occasion. I am told that the sand bags fell on the people and around 10 people are serious," he said.
State Devasom Minister Kadanapally Ramachandran informed about his plans to visit the accident site.
"I have spoken to the district administration and have been told that the toll is high and there are several people who have been injured also," Ramachandran told reporters.