Reporter Enda Brady says nothing could have been done to prevent the attack
British police say a lone male attacker carrying an improvised explosive device was responsible for a bombing at Manchester Arena.
The explosion at the end of a sold-out Ariana Grande concert has killed 22 people and injured 59 others.
Children are among those who died, Greater Manchester Police chief constable Ian Hopkins has confirmed.
Investigators are trying to establish whether the attacker, who also died at the scene, was part of a wider network.
The bombing is being treated as a terrorist incident, police said.
Sky News reporter Enda Brady is in Manchester. He told Pat Kenny how events unfolded.
"There were 60 ambulances last night, the injured are being treated at eight different hospitals across the Manchester region."
He says the Etihad Stadium, home of Manchester City football club, has opened its doors and is being used as a central point for families looking for information.
"A handful of families have come down here who are missing loved ones, they're being looked after - they're being given food and water and hot drinks - but the one thing they desperately need is information.
"It's harrowing watching people come in and out, and they're just pacing around - everytime their phone rings they're answering it within a split second - but nobody's getting any good news".
Grande's fanbase is primarily teenagers amid schoolchildren.
"She's a huge American star, (she) found fame on the Nickelodeon TV channel, which is a kids channel.
"20,000 people were at that concert last night, the majority will be schoolchildren - 14 and upwards would be her fanbase.
"Many of them will have been there last night with their parents - and from what I'm hearing in Manchester, many youngsters were given the tickets as Christmas presents".
"At precisely 10.33pm as the concert ended and people were filling towards the exit, this guy had come in via a public area... and the perpetrator had come in with his rucksack and quite deliberately timed it to people coming out of the exits".
"Security in these venues, it's big business... and it's very, very good.
"There's absolutely nothing that could have been done to prevent this last night.
"This is devious, calculated and timed to devastating effect."
Worried family and friends have been appealing for help to find loved ones who have not been in touch since the bombing.
There have been many appeals circulating on social media, and some of them are unconfirmed.
The 15-year-old's mother Charlotte told the Manchester Evening News she has not been able to reach her daughter since the explosion. She went to the Ariana Grande concert with her best friend, who is being treated at Manchester Royal Infirmary.
Olivia's uncle, Stephen Hodgson, said: "We've rung every hospital, hotel, we've been down to Manchester. We've been everywhere, we're just absolutely devastated. All we want to do is get a phone call. If anybody sees her please ring police, contact us. We're absolutely gutted as a family."
Chloe, 17, and her 19-year-old boyfriend were said to be at the gig. Their families have been in touch with Greater Manchester Police.
Courtney was believed to have been with her stepfather at the concert. On Facebook, her mother Deborah appealed for any information to help find them, writing: "I need them home safe."
Alison's daughter has appealed for information on social media, saying both were at the Manchester Arena.
Scottish politician Angus McNeil has said he is "very worried" about Laura MacIntyre and Eilidh MacLeod, who were in Manchester last night. Both of their families have appealed for help in finding the girls safe and well.
Kelly has not been heard from since the attack. Her friend said Kelly's partner has been driving around hospitals seeking information. It is believed she had fallen unconscious and was separated from her sister, who is being treated in hospital.
Her family say she has not been seen since just before the attack began at the Manchester Arena.
The 29-year-old has not been in touch with his family since the attack, and it is believed he had become separated from his friends at the concert.
Kaia is 16 and from France. She was attending the Ariana Grande concert alone. Kaia had been staying with friends in Manchester, and they have not heard from her since.
Alex Klis has appealed for help in finding her parents, who are missing.
The 14-year-old remains unaccounted for after last night's concert.
Meanwhile, there are reports a number of Polish citizens are also among the missing.
Additional reporting: IRN