Kumar was not present in the cavalcade, which was on the way to Gaya for his scheduled visit on Monday, they said.
The incident happened around 5 pm at Sohdi Mode in Gauri Chuk police station area when a road blockade was underway over the death of a local man, they added.
Soon, a police force was sent to the area and the mob was dispersed, he said.
"A case has been registered against 15 people and police have already arrested 11 people. We are analysing the CCTV footage, and will soon nab the remaining four persons," he added.