"Till 1 PM, 39.5 per cent voting was registered in the Mat seat where polling will continue till 5 PM", U P Chief Electoral Officer Umesh Sinha told PTI.
Mat seat is witnessing a keen contest among ruling SP, RLD and Trinamool Congress.
While SP has fielded Sanjay Lathar in a bid to retain Mat, RLD has named Yogesh Nauhar as it's candidate. Congress, which has an alliance with RLD, has not fielded its candidate.
Former minister Shyam Sunder Sharma is also trying his luck on a TMC ticket.
BSP and BJP have decided not to contest the by-election.
The by-election was necessitated after RLD general secretary Jayant Chaudhary resigned. He opted to retain Mathura parliamentary seat.