Such a horrific incident has drawn reactions of celebrities from across the globe.
In Singapore, the organiser of Britney Spears' concerts in June said they were considering new measures. "Please keep the victims & their families in your thoughts", he said. "This is clearly a concerning time for people but we are doing all we can to support those affected and gather information about what has happened".
Confirming the details of the attack, Chief Constable of Greater Manchester Police Ian Hopkins said the investigation to identify the perpetrators behind the deadly attack is underway, until which the attack is being treated as a terrorist incident.
The PM said: "It is now beyond doubt that the people of Manchester and of this country have fallen victim to a callous terrorist attack, an attack that targeted some of the youngest people in our society with cold calculation".
She said: "We now know that a single terrorist detonated his improvised explosive device near one of the exits of the venue, deliberately choosing the time and place to cause maximum carnage and to kill and injure indiscriminately".
Britain's political parties have agreed to suspend election campaigning until further notice following an explosion in Manchester that killed 19 people at a concert by USA singer Ariana Grande.
After the show finished witnesses described glass and metal nuts being found on the floor. Emergency services are on the scene of the incident. More than 400 officers were deployed after the blast. "My thoughts are with all those who have lost loved ones or been injured in this barbaric attack".
Nearby Victoria Station was closed and will remain closed for the entire day.
Another witness, who gave his name as Andy, told BBC Radio 5 he was waiting in the foyer for his wife and daughter when the explosion knocked him to the ground. Confusion followed as people fled in search of safety, and the sound of wailing sirens soon cut through the smoky air outside the arena.
The blast occurred around 10.35pm on Monday night at the conclusion of an Ariana Grande concert in the northern England arena.
Reuters news agency reported that two USA officials said initial signs pointed to a suicide bomber, although United Kingdom authorities have not confirmed the information. "We ran, and people were screaming around us and pushing on the stairs to go outside, and people were falling down, girls were crying, and we saw these women being treated by paramedics having open wounds on their legs".
Police cars, bomb-disposal units and 60 ambulances raced to the scene as the scale of the carnage became clear.
Ms Grande, 23, has said she is "broken" following the explosion moments after she left the stage on Monday evening.
Grande, who was not injured, tweeted hours later: "broken. from the bottom of my heart, i am so so sorry. i don't have words".
Meanwhile Grande's management team, Scooter Braun Projects, said they had been left heartbroken by the incident and praised the actions of Manchester's emergency services.
"My thoughts are very much with those who have been injured and lost their lives and their loved ones at this bad time, we are doing all we can to support them".
"As people are waking up to this tragic news on what is a sad day for Greater Manchester, the officers and staff from Greater Manchester Police and the other emergency services will continue to do all they can to help get us through the hard days ahead".
Footage appearing to show Ariana Grande's tour bus arriving back at her Manchester hotel has since been shared on social media.
"We regularly carry out exercises to test our ability to respond to such incidents and this has ensured a very swift response from local and national agencies". "There are a number of confirmed fatalities and others injured", police say.
"As part of the UK-wide response to these events, Police Scotland continues to review all safety and security plans and operations".