The districts where polling is on, amid tight security arrangements, are Saharanpur, Bijnor, Moradabad, Sambhal, Rampur, Bareilly, Amroha, Pilibhit, Kheri, Shahjahanpur and Badaun.
In the second of seven-phase election, 2.28 crore voters, including over 1.04 crore women, are eligible to cast their ballots in 14,771 polling centres and 23,693 polling stations.
Official sources said polling was dull initially but picked up as the day wore on.
They said till midday there was no report of any untoward incident and polling was going on smoothly.
"Polling is going on peacefully at all polling stations. We are keeping a close vigil. The poll percentage is likely to increase as the day proceeds," UP Chief Electoral Officer T Venkatesh said.
At certain places long queues were seen outside polling stations. First time voters were found to be enthusiastic, taking selfies proudly showing indelible ink imprint on their fingers.
Muslim women in burqas were seen outside several booths, while the picture of a newly married couple going to cast vote in Bareilly went viral on social media.
Elaborate arrangements have been made for free and fair polling with deployment of 51,771 police constables, 52,598 homeguards, 5,397 sub-inspectors and 3,666 head constables to maintain law and order.
Of the 67 seats at stake, ruling Samajwadi Party had won 34 in 2012, followed by BSP 18, BJP 10, Congress three and others two.
In this phase, 720 candidates are in the fray with a maximum of 22 from Barhapur (Bijnor) and a minimum of four nominees from Dhanaura (Amroha).
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.
Prime Minister Narendra Modi, Chief Minister Akhilesh Yadav, Congress vice president Rahul Gandhi and Bahujan Samaj Party chief Mayawati have campaigned extensively in these constituencies.
This time, Uttar Pradesh is witnessing a three-way contest among Samajwadi Party-Congress alliance, BJP and BSP.
While the Samajwadi Party seeks to retain power albeit with Congress prop, BSP and BJP hope to regain power in the state after five and 13 years respectively.
The Election Commission has barred political parties and candidates from publishing advertisements in newspapers on February 14 and 15 without its approval.
The poll body has asked them to get the advertisements pre-certified from the Media Certification and Monitoring Committee at the state and district levels before publication.
The decision has been taken in view of past instances of advertisements of offending and misleading nature in print media on such occasions which vitiate the elections.
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.