Two people have died following a party at a hotel in Cookstown, Co Tyrone, and a “multi-agency incident” is under way, the Police Service of Northern Ireland has said.
There were a “small number” of other casualties following the incident at the Greenvale Hotel, police added.
The hotel on Drum Road had been holding a St Patrick’s Day party for young people.
While the circumstances of what happened are still under investigation, there were reports of a crush at the scene, police said.
Sky News’ senior Ireland correspondent David Blevins said there were conflicting reports.
“One suggests that a number of young people collapsed on the dance floor of the hotel, while another suggests that after those young people collapsed there was something of a stampede by others to reach the door and there was a crush,” he said.
“It’s not clear yet what happened but what we do know is that the evening ended in the most tragic of circumstances.”
Shortly after the incident, police posted a message on Twitter urging parents to pick up their children from the venue.
Emma Heatherington, whose two sons, aged 17 and 18, had been at the event, said she “felt sick” when she saw the police appeal.
“My two boys were there so I rang them, shaking and thankfully got them both,” she said. “They were on their way home at that stage.
“They say there was a huge crowd waiting to get into the venue and then a lot of commotion up near the front of the queue, then they said the emergency services arrived and they were all asked to leave.
“They came home very, very shaken.”
The Northern Ireland Ambulance Service said it had taken four people to hospital.
“Three patients have been taken, by ambulance, to Antrim Area Hospital for treatment for various injuries and another to Craigavon Area Hospital.”
It said it had received several calls from about 9.30pm on Sunday and sent two rapid-response paramedics, two doctors and five emergency crews to the scene.
Assistant Chief Constable Mark Hamilton said: “I can confirm there are two fatalities at this stage and a small number of other casualties.”
He added: “While the exact cause of the incident is still unknown, there are reports of a crush at the scene.
“Police have commenced a full investigation into the circumstances.”
Democratic Unionist Party leader Arlene Foster tweeted that her “thoughts and prayers” were “with everyone impacted by this tonight”.
The DUP’s Lord McCrea told Sky News: “Young people go out and parents expect their children to come home, but some children aren’t going home. Irrespective of what the circumstances are, it’s an absolute tragedy.”
Colin Neill, chief executive of Hospitality Ulster, said: “We are devastated to hear of the confirmed PSNI report of at least two deaths this evening at the Greenvale Hotel in Cookstown.
“This is shocking and tragic news and our thoughts and prayers are with the families of the deceased.”