UK Prime Minister Theresa May issued a statement that said "we are working to establish the full details of what is being treated by the police as an appalling terrorist attack."
"All our thoughts are with the victims and the families of those who have been affected," she added.
May has also announced a temporary suspension of her campaign for her Prime Ministership, in which she faces a stiff challenge from Labor challenger and opposition leader Jeremy Corbyn.
Corbyn and Liberal Democrat candidate Tim Farron have also expressed their condolences and suspended their campaigns. For Farron, this meant canceling a planned campaign trip to Gibraltar.
On Tuesday morning, May will chair a Cabinet Office Briefing Room A (COBRA) government emergency response committee to the Manchester crisis.