Karen Ford had taken her daughter to see chart-topping American pop star Ariana Grande.
She said: "The reality has just sunk in that we went to a pop concert that was a very family affair and people have died there tonight."
Police have now confirmed 19 people have so far died and with around 50 others injured. A controlled explosion of another suspect device near the venue was carried out around 1.30am.
Greater Manchester Police confirmed: "Just before 10.35pm on Monday, May 22, police were called to reports of an explosion at Manchester Arena.
"So far 19 people have been confirmed dead with around 50 others injured.
"This is currently being treated as a terrorist incident until police know otherwise."
Sheffield concertgoer Karen Ford said they heard a huge bang just after the concert finished and they were making their way outside, where her husband was waiting to drive them home.
She said he believes he heard a second bang.
Karen said: "We were leaving the concert when there was a huge sound like an explosion.
"I only heard one bang but my husband said he heard a second bang outside.
"Everyone stopped and somebody said it was a bomb. Everyone was running and pushing.
"A lot of children were there without parents, screaming, pushing and shoving
"It was chaos. There were shoes on the floor where people had run and there shoes had fallen off. It was awful.
"I didn't think it was a bomb at first, it was just a huge bang in the left hand corner.
"When we were leaving we heard the awful news that people had died there tonight.
"I was telling people to calm down and telling people it was a balloon that had gone off, just to calm them down.
"I wouldn't be surprised if people had been crushed trying to leave the building.
"Outside there were children, many young teenagers, crying and parents crying as they were trying to find one another.
"A few people shouted it was a bomb and with emergency services arriving people knew there was something wrong.
"As soon as I met up with my husband we got away as quickly as possible."
Other witnesses reported hearing a “huge bang” in the foyer area at the venue after an Ariana Grande gig on Monday evening.
The area around the arena was swamped with police and emergency services. Approach roads were closed by police.
Manchester’s Victoria station, which backs onto the arena,was evacuated and all trains cancelled.
One fan at the concert described how the explosion hit the venue as fans were leaving the building.
Majid Khan, aged 22, said: “I and my sister, along with a lot of others were seeing Ariana Grande perform at Manchester Arena, and we were all exiting the venue when around 10.40-10.45pm-ish a huge bomb-like bang went off that hugely panicked everyone and we were all trying to flee the arena.
“It was one bang and essentially everyone from the other side of the arena where the bang was heard from suddenly came running towards us as they were trying to exit Trinity Way and that was blocked so everyone was just running to any exit they could find as quickly as they could.
“Everyone was in a huge state of panic, calling each other as some had gone to the toilet whilst this had gone off, so it was just extremely disturbing for everyone there.” Jade Baynes, 18, from Hull, was told to run from the area by armed police after leaving the concert arena.
She said she heard loud bangs and what sounded like gunshots just after the entertainment had finished.
She added: “There were just a loud bang and a flash and everyone tried to scramble out.
“An alarm came on telling everyone to stay calm but leave as quickly as possible.” Jade and her friend Jasmine Mia, 21, also from Hull, said there appeared to be some sort of commotion in the first tier of the arena, with a number of stewards stood around the seats.
Armed police swamped the streets around the arena and police are moving members of the public away from the area.