Polling in underway in Mathura, where BJP candidate and actress Hema Malini is pitted against sitting MP from RLD Jayant Chaudhary, which reported about 50 per cent turnout till 3 PM.
The trouble started when two persons belonging to RLD and BJP entered into an altercation on the issue of victory of their respective candidates, they said.
After casting their votes, they engaged in fisticuffs outside the polling booth. Later, the supporters of both the parties (RLD and BJP) resorted to brickbatting in Daulatpur village, followed by firing from both sides there, police said.
"Twelve persons have been arrested and there is no hindrance in the polling," SDM Vishwabhushan Mishra said.
Victims Bacchu Singh, Kare Singh and Om Vir have sustained internal and external injuries, police said.
Heavy security force has been put in the village and polling is going on smoothly, when the last report came in, police said.
Over 36.62 per cent voters exercised their franchise in the first six hours in Uttar Pradesh, where polling is going on in 12 seats in the third phase to decide fate of 188 candidates.