Voting has been going on throughout the day in West Tyrone as constituents elect a new MP.
The Westminster by-election was triggered by the resignation of Sinn Féin’s Barry McElduff in January.
Candidates from NI’s five main parties are represented on the ballot paper and 64,000 people are eligible to vote.
Police have arrested a 59-year-old man on suspicion of possessing a firearm with intent following an incident at a polling station in Greencastle.
Electoral officers requested additional police support after reports of an incident involving a man who appeared to be armed with a handgun.
At around 13:00 BST, the man walked into the polling station at Our Lady of Lourdes Primary School and revealed the suspected firearm to electoral staff.
It is understood members of the public were casting ballots at the time but that they were unaware of what was happening and no one was harmed.
Police in Omagh are investigating the incident and the man has been taken into custody for questioning.
Chief Electoral Officer Virginia McVea visited the polling station shortly after the incident to hear the details of what happened directly from the staff who witnessed it.
She told the BBC she was very impressed with how they conducted themselves, saying that that they “remained calm throughout” and “were mindful of other people in the room at the time”.
She added that voting at the polling station has not been interrupted.
The polls opened at 07:00 BST and will close at 22:00 on Thursday night.
The count will be conducted overnight with a result expected on Friday morning.
The five candidates are:
- Órfhlaith Begley, Sinn Féin
- Thomas Buchanan, Democratic Unionist Party (DUP)
- Stephen Donnelly, Alliance
- Daniel McCrossan, Social Democratic and Labour Party (SDLP)
- Chris Smyth, Ulster Unionist Party (UUP)
The West Tyrone constituency had a 68% turnout during the last Westminster election, one of the highest turnouts for the election.
When voting at the 82 polling stations ends, the ballot boxes will be brought to Omagh, County Tyrone where counting will begin.