As details continued to emerge about the outrage - the worst terrorist attack since 52 innocent people were killed in the July 7 bombings in London in 2005:
:: The first victim of the attack was named by her college as Georgina Callander;
:: Many of the 59 people injured are being treated for life-threatening conditions;
:: Investigators believe they know the identity of the bomber, and are working to establish whether he was acting alone or as part of a network;
:: Police said the investigation is "complex and wide-ranging" and cautioned against speculating on the attacker's identity;
:: Grande said she had been left "broken" by the events;
:: General Election campaigning has been suspended;
:: Andy Burnham, the Greater Manchester metro mayor, said the attack was an "evil act";
:: Scotland Yard announced it was stepping up the number of officers on duty around London in response to the bombing;
:: US President Donald Trump denounced those responsible for the atrocity as "evil losers" and pledged America's "absolute solidarity" with the people of the United Kingdom.
Latest on the terror attack in Manchester:
Speaking outside Downing Street after chairing a meeting of the Government's Cobra emergency committee, Mrs May said that police and security services believed they knew the identity of the bomber, who died alongside his victims.
She said that he had chosen the time and place of his attack deliberately to cause "maximum carnage and to kill and injure indiscriminately".
Mrs May, who was later travelling to Manchester to speak to police chiefs, paid tributes to emergency workers and members of the public who rushed to help.
She said they had shown: "The spirit of Manchester and the spirit of Britain - a spirit that through years of conflict and terrorism has never been broken and will never be broken."
Police were called to reports of an explosion at the Manchester Arena at 10.33pm on Monday, shortly after the end of the performance.
Victims described being thrown by the blast, which scattered nuts and bolts across the floor.
Mrs May 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.
"This was among the worst terrorist incidents we have ever experienced in the United Kingdom, and, although it was not the first time Manchester has suffered in this way, it is the worst attack the city has experienced and the worst ever to hit the North of England."
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."
Noting that the audience at the concert included many young children and families, the Prime Minister said: "All acts of terrorism are cowardly attacks on innocent people, but this attack stands out for its appalling, sickening cowardice, deliberately targeting innocent defenceless children and young people who should have been enjoying one of the most memorable nights of their lives."
Mrs May said parents and relatives of missing people would be experiencing "unimaginable worry" as she urged anyone with information relating to the attack to contact police.
Paying tribute to those who stepped in to help in the aftermath of the blast, the PM said: "While we experienced the worst of humanity in Manchester last night, we also saw the best. The cowardice of the attacker met the bravery of the emergency services and the people of Manchester.
"The attempt to divide us met countless acts of kindness that brought people closer together and in the days ahead, those must be the things that we remember."
Mrs May said there would be "difficult days ahead" for those affected, but added: "We all - every single one of us - stand with the people of Manchester at this terrible time.
"And today, let us remember those who died and celebrate those who helped, safe in the knowledge that the terrorists will never win and our values, our country and our way of life will always prevail."
The Prime Minister said the official threat level remains at severe - meaning an attack is highly likely - but this would be assessed throughout the day.
More on the incident in Manchester: