The Little Mix twitter account had shown no sign of trouble, encouraging fans to send in pictures of themselves looking forward to the concerts.
Families had taken advantage of the Easter holidays to travel from Belfast, Downpatrick, Coleraine, Derry and further afield.
It wasn't until 1.25pm that security guards came outside holding signs to make the announcement that the concert was cancelled after one of Little Mix's four singers, Jesy Nelson, had taken sick.
Aiken PR, which was handling promotion for the concerts, hastily issued a statement.
It read: "Little Mix have unfortunately had to cancel both of their Arena shows in Belfast today. Sadly Jesy is unwell and unable to sing, and as a result both the matinee and evening shows are cancelled.
"Little Mix would like to apologise to their fans. Ticket holders should hold on to their tickets and await further news."
The girl group is next scheduled to perform in Sheffield on Saturday night and no concerts have been planned in the SSE arena for tonight. As yet it's not known if the concerts can be rescheduled that quickly.
With the infamous cancellation of a One Direction pop concert in the SSE Arena last year, many parents were left wondering what happened to the old saying "the show must go on".
There was also anger that it had been left to security guards to make the announcement.
Many contacted the Belfast Telegraph to express their disappointment.
Carolyn McWilliams said: "I am gutted that tonight's concert has been cancelled as I purchased the tickets for my daughter's 10th birthday, which is today.
"We didn't have a party or anything as she was attending the concert and now we have nothing."
Stephen Dobbins from Bangor had travelled with his nine-year-old daughter Emily.
"Emily is heartbroken," he said. "This was her first concert, she was looking forward to going with her mum Karen to see her favourite band."
Tara Clelland had planned to go to the evening concert.
She said: "I was taking my 10-year-old tonight - she is devastated."
Sarah Delaney said: "My four-year-old daughter had tickets today to see Little Mix as a Christmas present. We stayed over the night before as my son was seeing The Vamps. She is so upset but it's understandable if someone is sick. Just hope we are able to go when it's rescheduled."
Robert Daly travelled from Galway with his fiancée while her family flew in from Liverpool.
"Very disappointed," he said. "This was a Christmas gift!"
Emma Begley (33, left) and daughter Gracey (8) flew in from Wales just for the concert,
"It's devastating," Emma said.
"It was quite a scene at the Odyssey. I could only get two tickets which I had to get from Gumtree.
"It's supposed to be my daughter's first concert. She's crestfallen; her whole trip was for that concert."
She added: "We're hoping for a reschedule for tomorrow. There could be a chance.
"They're in Sheffield in Saturday and nowhere tomorrow so we're crossing our fingers."
Gracey, who spent yesterday afternoon in tears listening to Little Mix on her iPod, said: "I felt really sad. We were having our lunch and then we went to the queue they said Jesy was sick. It's really disappointing because we flew from Wales just to see them."