SP Rajaram Rajasekharan said all the casualties occurred on board the tractor trailer, which overturned as well as the lorry.
"Four persons died on the spot when they were crushed under the overturned tractor trailer. Eight others died in the hospital," he said.
Local people rescued the injured labourers and rushed them to hospital.
The injured were admitted to the Kalna Sub-Divisional Hospital where the condition of many was stated to be critical, police said.