Shortly after 22:35 BST, the streets of the city centre were suddenly lit up in blue flashing lights, and helicopters buzzed overhead.
It was obvious from the beginning that this was incredibly serious.
Closer to the arena, dozens of people could be seen streaming away in confusion, unsure what they were running from.
Once at a safe distance, some stopped to sit and rest, many of them in tears, and to telephone relatives.
Manchester Arena blast: 19 dead and more than 50 hurt
Manchester explosion: Latest updates
'Some of the people looked dead'
One mother who attended the concert by US singer Ariana Grande told the BBC her priority was simply getting her daughters home.
Other people were more candid, and spoke of seeing people left "covered in blood", as they desperately tried to flee a scene of "chaos".
Coats and mobile phones were reportedly left strewn on the arena floor as people dumped their possessions and ran.
Paramedics were quickly at the scene, treating the injured.
College student Sebastian Diaz, 19, travelled to the concert from Newcastle.
"Ariana Grande had just finished her last song and there was a huge bang," he said. "I just saw running and it was just instinct to run with them.
"We actually ended up in a corridor and it was a dead end. It was terrifying.
"I found the main doors and people were crying everywhere. Back at the hotel people were crying and on their phones."
Calvin Welsford, 18, travelled to the concert from Bristol, and described initially running down a corridor and also hitting a dead end.
He said: "I was in the front row and it seemed to happen at the very back of the arena.
"It almost sounded like a gunshot. That's how I would describe it. It was very loud.
"I looked around and people were just spilling down, heading out of the building.
"It was sheer panic. Security told us we needed to leave but they were very caring. People were crying."
A wide area around the venue was quickly cordoned off, and the crime scene later appeared to widen with police pushing people further and further back.
Police officers cut a calm and reassuring tone, and urged people to move away.
One thing that has been apparent is the many young people who had attended the event, some of them with parents or guardians.
There was a huge sense of confusion with dozens of people seen searching for information on their phones.
Some shops and takeaways in the busy city centre area around the Arena appeared to be handing out drinks, while taxis swarmed to the area to help take people away.

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The 29-year-old scored a single yuko to beat Graf 1-0 in the first bronze medal match in the weight category.
Conway had earlier beaten world champion Gevrise Emane of France but was beaten by Colombia's Yuri Alvear for the right to fight for gold.
The Bristol-born judoka won bronze for Scotland at the 2014 Commonwealth Games.
"It hasn't sunk in," she told BBC Sport. "I knew if I was going to come away with a medal I would have to put in my best performances.
"I'm not from Brazil but the crowd have been awesome. I just want to thank everyone."
Find out how to get into judo with our special guide.
Conway suffered shoulder ligament damage during her second-round defeat at London 2012 and had to have surgery.
"This has been my dream since London," she added. "I can't put it into words."
Great Britain's Gemma Gibbons, who won judo silver at London 2012, tweeted: "She's only gone and done it.
"Absolutely amazing from Sally Conway. Our Olympic medallist Miss Conway."
Japan's Haruka Tachimoto beat Colombia's Yuri Alvear by ippon to win the gold medal and Germany's Laura Vargas Koch defeated Spain's Maria Bernabeu by wazari to win the other bronze medal contest.
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People have described the "sheer panic" of the Manchester Arena blast that left 19 dead, more than 50 injured and survivors fleeing for their lives.
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Briton Sally Conway defeated Austria's Bernadette Graf to win the -70kg judo bronze medal.