“With regards to last night’s incident at the Manchester Arena, we can confirm we have arrested a 23-year-old man in south Manchester,” Greater Manchester Police said.
Two victims have been identified so far: eight-year-old Saffie Rose Roussos and 18-year-old student Georgina Callender.
At least 22 people were killed on Monday night when a man detonated a device in the foyer area of the Manchester Arena at the end of a pop concert, injuring 59 others.
It is feared the death toll will climb as some of the patients are in critical condition, 12 of them under the age of 16, healthcare services said.
Greater Manchester police also said they have “executed warrants, one in Whalley Range and one in Fallowfield, where a controlled explosion took place as part of the investigation into last night’s horrific attack”.
Meanwhile, terror group Daesh apparently claimed responsibility for the killings in a web statement.
The arena had been hosting a concert by U.S. pop star Ariana Grande and thousands of children and teenagers were among the crowd.
Prime Minister Theresa May said the identity of the perpetrator was known to police but added the authorities would not be revealing more information for the moment.
She said many of the 59 casualties being treated across eight regional hospitals had life-threatening injuries.
All campaigning ahead of the country’s June 8 general election has now been suspended, May added.
'Appalling and Callous'
May, speaking after an emergency meeting of British ministers and intelligence figures, described the blast as “appalling and callous”.
“This was amongst the worst terrorist incidents witnessed in the U.K.", she added.
Shortly after May spoke, police evacuated a major shopping center in central Manchester, amid an alert.
May will travel to Manchester later on Tuesday and another emergency COBRA meeting will be held.
Police had earlier said the blast was the work of one man but added their “fast-moving investigation” involving 400 officers was trying to determine if he had acted alone or was part of a wider plot.
Monday night’s blast at the 21,000-seat Manchester Arena took place shortly before 10.35 p.m. (2135GMT) local time.
The arena had been hosting a concert by U.S. pop star Ariana Grande and thousands of children and young teenagers were among the crowd.
Turkey condemned the attack with President Recep Tayyip Erdogan saying the country shared the grief of the British government and its people.
"Like every country, I would like to stress that we stand with Britain in the fight against terrorism," Erdogan said.
Other leaders have also condemned Monday’s attack.
Speaking in Bethlehem during a visit to the Middle East, U.S. President Donald Trump offered his condolences to the British people.
“We cannot stand a moment longer to the slaughter of innocent people,” he said.
The Kremlin released a statement, saying Russian leader Vladimir Putin condemned the “inhuman, cynical” attack.
French President Emmanuel Macron is to call the U.K. prime minister later on Tuesday, the Elysee confirmed.
In the U.K. Manchester mayor Andy Burnham described the incident as an "evil attack" while main opposition party leader Jeremy Corbyn said he was "horrified" by the deaths.
Burnham said a vigil would be held at Manchester’s central Albert Square on Tuesday evening.
"This is horrific. This is criminal. May the perpetrators face full weight of justice both in this life and the next," a statement by the Muslim Council of Britain added.
The current terror threat level in the U.K. has been at severe, which means a terror attack is highly likely.
This incident is the worst terror incident on British soil since a series of deadly attacks in the capital London on 7 July 2005, which killed 52 people.