Greater Manchester Police have arrested a 23-year-old man in relation to Monday night's attack at the Manchester Arena, which left at least 22 people dead and 59 injured.
Another man died after detonating a device near the foyer of the venue after a concert by the singer Ariana Grande.
Prime Minister Theresa May confirmed that police believe they knew the identity of the attacker, but it has not yet been confirmed whether he acted alone.
Following the naming of teenager Georgina Callander as the first victim of the attack, police confirmed that a man was in custody.
The 23-year-old man was arrested in South Manchester.
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The Queen, meanwhile, has described the attack as "an act of barbarity".
"The whole nation has been shocked by the death and injury in Manchester last night of so many people, adults and children, who had just been enjoying a concert," she said in a statement.
"I know I speak for everyone in expressing my deepest sympathy to all who have been affected by this dreadful event and especially to the families and friends of those who have died or were injured.
"I want to thank all the members of the emergency services who have responded with such professionalism and care.
"And I would like to express my admiration for the way the people of Manchester have responded, with humanity and compassion, to this act of barbarity."
