The constituencies are spread across 11 districts -- Saharanpur, Bijnor, Moradabad, Sambhal, Rampur, Bareilly, Amroha, Pilibhit, Kheri, Shahjahanpur and Badaun.
Of the 67 seats at stake, ruling Samajwadi Party (SP) had won 34 in 2012, followed by BSP 18, BJP 10, Congress 3 and others 2.
While Prime Minister Narendra Modi and BJP president Amit Shah championed the cause of the saffron party during the campaigning, the SP-Congress alliance's charge was spearheaded by Chief Minister Akhilesh Yadav and Congress vice-president Rahul Gandhi, besides other leaders of the respective parties.
Both Modi and Shah slammed the Congress and SP, saying Rahul ran a campaign against the SP government and wondered what change of heart made them embrace each other now.
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Akhilesh and Rahul harped on demonetisation, alleging that Modi's note ban decision had caused immense hardship to those living in the rural areas.
Keeping in mind that western Uttar Pradesh had vast tracts of agricultural land, Modi played the pro-poor and pro-farmers card and reiterated BJP's promise of waiving loans of small and marginal farmers and paying the dues of sugarcane growers within 14 days of coming to power.
alleging that the Akhilesh government had let loose a "reign of terror" in Uttar Pradesh with hooligans committing "utmost atrocities" against women.
The BSP supremo also accused the Modi government of "interfering" with the personal law of Muslims and ending reservations for backward communities in jobs and promised to extend quota benefits to the poor among the upper castes.
Her refrain was that if Muslims wanted to defeat the BJP, they should not waste their votes by backing the SP-Congress alliance.
Development, clearance of cane dues and law-and-order were among the key issues raised by various leaders in the 67 constituencies, with every political party promising to address them if voted to power.
Prominent contestants include controversial SP minister Azam Khan and his son Abdullah Azam, who are contesting from Rampur and Swar seats respectively, Saif Ali Naqvi, son of former Congress MP Zafar Ali Naqvi, former central minister Jitin Prasada from Tilhar (Shahjahanpur), BJP Legislature Party leader Suresh Kumar Khanna from Shahjahanpur city and state minister Mehboob Ali from Amroha.
The fate of these candidates will be decided by 2.28 crore voters, including over 1.04 crore women.
The next five phases of polling will be held on February 19, 23, 27 and on March 4 and 8. Counting of votes will be taken up on March 11.