Britain is on critical terror alert with military troops set to bolster police forces amid fears Manchester attacker Salmon Abedi did not act alone.
Prime Minister Theresa May has raised the threat level to the highest possible rating, meaning another atrocity is expected imminently.
She said a "wider group of individuals" could have been involved in the Manchester Arena blast rather than just suicide bomber Abedi.
In a sign of the increased threat, the military could be deployed to support armed police officers, Mrs May added during a live televised statement from Downing Street.
Monday night's attack at a concert by US pop star Ariana Grande left 22 people dead, including an eight-year-old girl, and dozens injured.
As counter-terrorism agencies mounted a massive inquiry into the outrage - the worst terrorist attack since 52 innocent people were killed in the July 7 bombings in London in 2005:
:: Police have arrested a 23-year-old man near a Morrisons in Chorlton, south Manchester, in connection with the inquiry
:: Prime Minister Theresa May vowed that the "spirit of Manchester and the spirit of Britain is far mightier than the sick plots of depraved terrorists"
:: The first victims were confirmed as eight-year-old Saffie Rose Roussos from Leyland and teenager Georgina Callander from nearby Chorley
:: Many of the 59 people hurt in the attack were treated for life-threatening injuries. Twelve of those rushed to hospital were children
:: Donald Trump denounced those responsible for the atrocity as "evil losers" and pledged America's "absolute solidarity" with the people of the UK
:: Andrew Parker, the Director General of MI5, condemned the "disgusting attack" and declared that the agency remains "relentlessly focused" on tackling the "scourge of terrorism"
The Islamic State terror group claimed responsibility for the barbaric attack, which involved a home-made device packed with nuts and bolts which exploded in the venue's foyer as thousands of young people were leaving.
Abedi, believed to have been born in Manchester and of Libyan descent, has been named as the suicide bomber.
The 22-year-old studied business at Salford University but dropped out before completing his degree.
He is thought to have attended the Manchester Islamic Centre, also known as Didsbury Mosque, along with his parents and siblings.
A family friend, who asked not to be named, described him as "normal" and said they were known to the Libyan community in the city.
He told the Press Association: "He was always friendly, nothing to suggest (he was violent). He was normal, to be honest."
Abedi was named after armed officers carried out a raid and controlled explosion at an address in south Manchester where he was registered as living.
Elsewhere in the city, the first arrest was made in connection with the inquiry when a 23-year-old man was detained near a Morrisons in Chorlton.
The terror threat level was increased after investigations revealed he may not have acted alone.
Mrs May said Operation Temperer - allowing military personnel to take to the streets - is now in force.
She will chair a meeting of the Government's emergency Cobra committee at 9.30am on Wednesday.
Meanwhile, fears were growing for Chloe Rutherford, 17, and Liam Curry, 19, a couple from South Shields, 15-year-old Olivia Campbell from Manchester, Eilidh MacLeod, from Barra in Scotland, Kelly Brewster from Sheffield, and Martyn Hett and Wendy Fawell.
All were believed to have been at the concert and have not been traced since the attack.
The death of Saffie Rose Roussos, the youngest known victim of the attack, was described by her headteacher as "heartbreaking".
Chris Upton, of Tarleton Community Primary School, said: "Saffie was simply a beautiful little girl in every aspect of the word."
Another victim was named by her college as Georgina Callander, who was studying health and social care at Runshaw College in Leyland, Lancashire.
Tributes were also paid to 26-year-old John Atkinson from Bury, who was named by friends on Facebook as an apparent victim.
The country's senior anti-terror police officer said there were "gaps in our knowledge" about Abedi which had led to the increased threat level.
Metropolitan Police Assistant Commissioner Mark Rowley said: "We are moving at pace, we are making arrests, we are doing searches but - not unsurprisingly - there are still gaps in our knowledge.
"Whilst we are chasing those gaps down, on a precautionary basis, based on that judgment, JTAC (the Joint Terrorism Analysis Centre) have made this judgment about the threat level and we will respond in our policing stance to that decision."