Jessica Lawrence was among the crowd gathered inside the Royal Arena where the former One Direction star was due to perform one of his Love on Tour shows on Sunday evening.
“Everyone was kind of jovial – clapping around, singing along to the music then at around 9 o’clock the mood really kind of shifted when people realised what was going on,” she told BBC Radio Ulster.
Police in Denmark said three people were killed and four others seriously hurt after the attack which happened in Field’s - one of the biggest shopping centres in Scandinavia.
Read more: Gunman kills three, leaves three critically wounded at Denmark shopping centre
Read more: Several people killed in shooting at shopping centre in Copenhagen, Denmark
A 22-year-old Danish man was arrested after the shooting, Copenhagen police inspector Soren Thomassen said. The suspect has been charged with manslaughter and will face questioning in front of a judge on Monday.
BBC NI journalist Ms Lawrence said the Harry Styles gig had been due to start at 8pm but had been pushed back due to the shooting attack at the shopping centre.
However, moments before Harry was expected to appear on stage shocked fans were told the arena was being evacuated, with fans receiving a police escort.
“Someone came out on stage to say ‘we’ve been in contact with police – they say the show is ok to go ahead but we are just going to delay by around an hour’.
“Then someone came on stage around half nine to say ‘due to safety concerns the concert is cancelled, we’ll now start evacuating you out – the police are outside to make sure that it all runs smoothly.”
Ms Lawrence described the eerie atmosphere as she and thousands of others made their way to safety.
“As we were kinda walking to the Metro station you could definitely tell from the concert goers that something really, really terrible had happened.
“I think a lot of people were in shock.”
The singer later took to social media to express his own shock at what had happened and urged fans to look out for one another.
"I’m heartbroken along with the people of Copenhagen. I adore this city. The people are so warm and full of love,” he tweeted.
“I’m devastated for the victims, their families, and everyone hurting.
“I’m sorry we couldn’t be together. Please look after each other. H”
Copenhagen police inspector Mr Thomassen told reporters, adding there was no indication that anyone else was involved in the attack, police were still investigating.
He later said the gunman most likely acted alone and selected his victims at random.
Investigators do not believe the attack was terror-related.
Gun violence is relatively rare in Denmark.