Brisk to moderate polling was recorded till noon, officials here said, adding that no untoward incidents have been reported so far.
The counting of votes will take place on July 11, they said.
In the bye-election, Bahuguna is locked in a straight contest with BJP's Prakash Pant, who has been a Cabinet minister as well as Speaker of the Assembly during the tenure of previous BJP governments.
Pant lost the recent Assembly election from Pithoragarh early this year.
The Sitarganj seat fell vacant after former BJP MLA Kiran Mandal quit the seat in favour of Bahuguna "for the sake of development of his constituency."
Both the main political parties -- Congress and BJP, relied heavily on the state leadership for campaigning.
Along with state PCC Chief Yashpal Arya, Bahuguna himself shouldered the responsibility of campaigning and the two leaders were camping in the constituency for most of the time when electioneering began.
On the BJP's side, the campaign was spearheaded by former Chief Minister and BJP national vice president Bhagat Singh Koshiyari, state BJP President Bishan Singh Chufal and leader of the opposition Ajay Bhatt.
Nearly 1,500 personnel, including four companies of central paramilitary force and three platoons of PAC, have been deployed in the constituency for security purpose.
Besides, polling staff comprising nearly 600 people have been put on duty on 116 polling booths, chief electoral officer Radha Raturi said.
There are a total of 91,247 electorates, including 42,447 women voters, in Sitarganj constituency.
