"Policemen were posted there and it should not have happened. A report has been sought on how the lapse occurred," director general of police Jacob Punnoose told PTI.
However, he said the car, driven by a local man, had not reached anywhere near the prime minister's car. The pilot vehicles of the convoy noticed the car and turned it away. The convoy had to be halted for one minute, he said.
On enquiry, it was found that the man was going to a nearby shop.
The convoy was proceeding from Kuttanellur helipad to Thrissur town hall where the prime minister paid homage to veteran Congress leader K Karunakaran.
The six-km road between Kuttanellur and town hall had been blocked by the police for the safe passage of the convoy.
However, when it reached the Nadathara junction, the private car ran into the main road causing embarrassment to the Special Protection Group and the police.