Superintendent of Police (Ghazipur) Yashveer Singh said the protesters were workers from the Rashtriya Nishad Party who were prevented by the police from going to the venue.
When the Prime Minister had left Ghazipur, party workers blocked traffic at various places and started pelting stones on the vehicles returning from the programme venue," he told PTI.
Constable Suresh Vats (48) was hit on the head by a stone when he went to end the traffic jam.
"He was immediately taken to a hospital where he succumbed to his injuries, the SP said.
Around 15 party workers have been detained and the rest of the protesters are being identified with the help of videography," he added.
The constable was from Raniganj in Pratapgarh district.