Balloting for one Lok Sabha and 11 assembly constituencies saw a moderate turnout by Saturday afternoon even as supporters of rival parties clashed at one place.
Officials said 45.60 percent of the electorate had voted in the Mainpuri Lok Sabha constituency, while 33.73 percent of the electors had voted in the 11 assembly seats till 1 p.m.
The highest voting turnout in the Thakurdwara, while the lowest - 16 percent till 1 p.m. - was recorded in Noida assembly constituency.
At a polling booth in Saharanpur, supporters of the Bharatiya Janata Party (BJP) and the Congress clashed during which a person suffered a heart attack and died.
Officials rushed to the spot, pacified the people and took the body for a post-morterm examination.
In another incident, BJP's ally Apna Dal MP Anupriya Patel and SP supporters were engaged in a verbal spat at a polling booth in Rohania.
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Samajwadi Party (SP) chief Mulayam Singh Yadav Saturday said his party will sweep the polls and will win the Mainpuri Lok Sabha and all the 11 assembly seats.
Mainpuri with 1,647,176 voters is crucial for the SP as the seat has been held by the party for over two decades.
It was vacated by Mulayam Singh Yadav after which the SP fielded Tej Pratap Singh Yadav, a grandnephew of Mulayam Singh, while the BJP has fielded Prem Singh Shakya. The Bahujan Samaj Party has this time decided not to contest.
Polling was very slow in Noida and only 17 percent of the 523,762 electorate had cast their votes in the first five years, officials said.
Other than Noida, by-polls are underway in Lucknow (East), Charkhari, Sirathu, Balha, Rohania, Nighasan, Saharanpur city, Bijnore, Thakurdwara and Hamirpur assembly constituencies.
Voting in the biggest assembly segment in terms of voters in the bypolls - Noida (523,762) and the smallest Thakurdwara (329,438) - was also under way peacefully.
"...we want more and more voters to come out and cast their votes," UP's Chief Electoral Officer Umesh Sinha told IANS.
In all, 3,820 micro-observers have been deployed across the 5,939 polling booths of which 1,920 have been declared sensitive.