The districts where polling is being held on Thursday are Ambedkar Nagar, Balrampur, Siddharth Nagar, Basti, Sant Kabir Nagar, Maharajganj, Gorakhpur, Kushinagar, Deoria and Ballia.
In this round, the polls will seal the fate of political heavyweights such as chief minister Yogi Adityanath, UP Congress president Ajay Kumar Lallu and former minister Swami Prasad Maurya.
These 57 assembly segments, of which 11 are reserved, are particularly crucial for the BJP.
In the 2017 assembly polls, it had won 46 of the 57 seats.
Among the 676 candidates in the fray this time are Yogi Adityanath from Gorakhpur. He is contesting his first assembly polls.
The Samajwadi Party has pitted the widow of late Upendra Dutt Shukla, a former BJP leader, against Adityanath. Azad Samaj Party founder Chandrashekhar Azad is also a contesting against the chief minister.
UPCC president Ajay Kumar Lallu from Tamkuhi Raj seat and Swami Prasad Maurya, who quit as BJP minister to join the Samajwadi Party, from Fazilnagar, are other prominent candidates in this phase.
Leader of Opposition in the outgoing assembly and Samajwadi Party leader Ram Govind Chaudhary is contesting from Bansdih. Many incumbent ministers' electoral fate will also be decided in this phase.
They include Surya Pratap Shahi from Pathardeva seat, Satish Chandra Dwivedi from Itwa, Jai Pratap Singh from Bansi, Shree Ram Chauhan from Khajani and Jai Prakash Nishad from Rudrapur.
Surendra Singh, the incumbent MLA who joined the Vikassheel Insaan Party (VIP), after being denied ticket by BJP is challenging BJP minister Anand Swarup Shukla from Bairia in Ballia.
The campaigning in this phase witnessed the political parties launching an all-out attack on each other.
Prime Minister Narendra Modi had attacked the BJP's rivals by tagging them as 'pariwarvadi' (staunch dynasts) who, he claimed, can never make India capable or Uttar Pradesh empowered.
Congress general secretary Priyanka Gandhi rebutted by saying the BJP was only against her family, which has not bowed down to the saffron party.
Samajwadi Party chief Akhilesh Yadav had accused the BJP of hatching a conspiracy of ending reservation and 'selling' government organisations to the private sector.
Meanwhile, there are a total of over 2.14 crore voters in this phase.
Over 868 companies of central paramilitary forces and state armed police, over 1.27 lakh civil police personnel, home guards and Prantiya Rakshak Dal (PRD) jawans and village guards, have been deployed for free and fair polling in the sixth phase of UP assembly polls on Thursday.
Additional director general (ADG), law and order, Prashant Kumar said polling will be held for 57 assembly constituencies in 10 districts.
Kumar said strategic deployment of forces has been done for sensitive constituencies, vulnerable areas, polling centre, booths, inter-state and international barriers. He said quick response teams, flying squads and static squads too have been deployed to ensure smooth conduct of elections.
"There are 9 sensitive constituencies in the sixth phase including Gorakhpur (city), Banshi, Itwa, Dumariyaganj, Ballia (city), Phephna, Bairia, Sikandarpur and Banshdeeh. Overall, 824 localities are vulnerable and 2,962 polling booths are critical," he added.