Congress candidate Vivek Dhakad won the Mandalgarh assembly seat by defeating his nearest rival, a BJP candidate, by nearly 13,000 votes.
After the 11th round of counting, 41-year-old Dhakad surged ahead of BJP candidate Shakti Singh Hada, who was leading in initial trends, and emerged as the winner.
He garnered 70,146 votes, defeating Hada by 12,976 votes.
Dhakad, a law graduate, had contested assembly election in 2013 as a Congress candidate from this seat but lost to Kumari by 18,540 votes.
Mandalgarh has traditionally been a Congress seat but last time the BJP had won the seat.
The Congress Party is leading on both the Lok Sabha seats from Alwar and Ajmer.
All the three seats fell vacant after the deaths of Ajmer MP Sanwar Lal Jat, Alwar MP Chand Nath and Mandalgarh MLA Kirti Kumari.
All the three leaders were from the Bharatiya Janata Party (BJP).
More details about the bypoll and candidates here
Here are the early trends of bypoll results: