The devastating concert bombing attack in Manchester on 22 May 2017, which claimed the lives of 22 people and left dozens injured and maimed, forced the British government to raise the United Kingdom's domestic threat level to the highest possible status, "critical".
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.
"We have to put our arms around them and support them, not just today but in all the very hard days to come because a trauma like this doesn't go in a day or two - it's there with them for the rest of their lives".
"We know that every terrorist attack on our country is an attack on every single one of us, but this is of a different order". It is a basic human right to be able to go out into public places and into public spaces, to shop, to go to a concert, to do our business, to take our exercise. "Keeping Australia safe is our first priority, as it is keeping Britain safe the first priority of Prime Minister May", he said. Emergency workers rushed to the arena, towards the danger, as concert goers fled from it. Public transport and transport hubs, sporting venues and places of mass gathering have been targeted.
"It is a possibility we can not ignore that there is a wider group of individuals linked to this attack", May said.
This would be the deadliest attack in the United Kingdom since the London bombings of July 2005.
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The Scottish National Party (SNP) has postponed the launch of its election manifesto, which was due today, and SNP leader Nicola Sturgeon said the news was "heartbreaking" and her thoughts were with the victims of the "barbaric" attack. "As parents we keenly feel the anxiety of those waiting to learn of their children's safety on this awful night", he said. This is an attack on innocence.
Noting that the audience included many young children and families, the PM 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".
Already, Mr Speaker, we are seeing the stories of bravery, solidarity and compassion emerging from this tragedy, as people rallied without a second thought to help each other. On the streets and online, beds, phone chargers, cups of tea were being offered to those caught up in the attack.
Our nations will never give in to terror.
A lone bomber detonated a homemade device packed with nuts and bolts in the foyer of the Manchester Arena as thousands of young people were leaving a concert by USA pop star Ariana Grande on Monday night.