Britain is on critical terror alert with military troops set to bolster police forces amid fears Manchester attacker Salman 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, armed troops could be deployed on the streets to guard concert venues and sports stadiums under a plan authorised by the Government in the wake of the Manchester Arena atrocity.
Soldiers will replace armed police at many sites under Operation Temperer, which is being enacted after security experts warned the Government that another terrorist attack could be imminent.
The decision taken at a meeting of the Government's emergency Cobra committee will mean soldiers could play a key role in protecting civilians and free up armed police officers to help fight the terror threat.
Operation Temperer, which was first revealed in 2015, is believed to allow up to 5,000 troops to be deployed in support of the police.
Prime Minister Theresa May said the operation had been authorised by Defence Secretary Sir Michael Fallon at the request of the police after experts at the Joint Terrorism Analysis Centre (JTAC) raised the threat level from "severe" to the highest "critical" setting.
"This request is part of a well-established plan, known as Operation Temperer, in which both the armed forces and the police officers involved are well-trained and well-prepared to work in this kind of environment," she said.
"The Secretary of State for Defence has approved this request, and Operation Temperer is now in force."
"This means that armed police officers responsible for duties such as guarding key sites will be replaced by members of the armed forces, which will allow the police to significantly increase the number of armed officers on patrol in key locations.
"You might also see military personnel deployed at certain events such as concerts and sports matches, helping the police to keep the public safe."
In a speech after chairing a meeting of Cobra last night, Theresa May said: "I ask everybody to be vigilant and to co-operate with and support the police as they go about their important work.
"I want to end by repeating the important message I gave in my statement earlier today.
"We will take every measure available to us and provide every additional resource we can to the police and the security services as they work to protect the public.
"And while we mourn the victims of last night's appalling attack, we stand defiant.
"The spirit of Manchester and the spirit of Britain is far mightier than the sick plots of depraved terrorists, that is why the terrorists will never win and we will prevail."
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
:: Mrs May vowed that the "spirit of Manchester and the spirit of Britain is far mightier than the sick plots of depraved terrorists"
:: Among the first victims to be named were eight-year-old Saffie Roussos from Leyland and teenagers Olivia Campbell, 15, from Bury and Georgina Callander from 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, Eilidh MacLeod, from Barra in Scotland, 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, 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."
On Tuesday Charlotte Campbell launched a desperate bid to find her daughter and made several emotional appeals on television.
She later said on Facebook: "RIP my darling precious gorgeous girl Olivia Campbell taken far far to soon go sing with the angels and keep smiling mummy loves you so much."
Another victim was named by her college as Ms Callander, who was studying health and social care at Runshaw College in Leyland, Lancashire.
Kelly Brewster, from Sheffield was reported missing by relatives who appealed for help finding her.
Her partner, Ian Winslow, later said on Facebook: "Kelly Brewster wasn't one of the unidentified hospital patients. She has sadly passed away in the terror attack yesterday.
"Kelly really was the happiest she has ever been and we had so many things planned together. My daughter Phoebe will be absolutely devastated like we all are."
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."
Greater Manchester Police Deputy Chief Constable Ian Pilling said the raised level will support the "significant" resources the force has in place.
He also praised the "tremendous strength and resilience" shown in Manchester on Tuesday, adding: "We need this to continue in the difficult days ahead."