British Prime Minister Theresa May has said the country’s terror threat level has been raised to its highest level of "critical," meaning an attack may be imminent.
She made the announcement late on May 23, a day after a suicide bomber detonated an explosive device after a concert in Manchester, killing 22 people.
Bombing suspect Salman Abedi may have been part of a bigger network, the prime minister said, adding that the man was born and raised in Britain.
She also said the government had triggered Operation Temperer, an emergency plan to put soldiers into key public locations to support armed police.
Manchester police said they arrested a 23-year-old man in connection with the Manchester attack. The extremist group Islamic State (IS) has claimed responsibility, though U.S. officials said they could still not confirm the group’s claim.
Thousands of people gathered at Manchester’s Albert Square to mourn the 22 killed, many of them children or teenagers.
Bathed in the evening sunshine, the packed square fell silent for a minute after several civic officials and poet Tony Walsh praised the city's spirit and solidarity in the face of the deadly attack.
Other cities around the world followed suit. In Rome, the city said lights at the ancient Colosseum, Trevi Fountain, and city hall would be turned off to honor the victims. Officials in Paris said the Eiffel Tower would do the same, while at the Cannes Film Festival, a moment of silence was held to honor the victims. New York City said it has heightened security at high-profile locations.
UEFA, European soccer's ruling body, announced that it would hold a minute of silence before Manchester United plays Ajax in Stockholm in the Europa League final on May 24 and that the opening ceremony would be "considerably reduced."
Speaking earlier on May 23 after chairing an emergency response meeting in London, May said police and security services were "working at speed to establish a clear picture" of what she called a “callous terrorist attack.”.
May said that there were "many children and young people" among the dead and injured, and that many of the 59 injured victims at eight hospitals across Manchester were being treated for life-threatening injuries.
"We now know that a single terrorist detonated his improvised explosive device near one of the exits of the venue, deliberately choosing the time and place to cause maximum carnage and to kill and injure indiscriminately," May said.
"We struggle to comprehend the warped and twisted mind that sees a room packed with young children not as a scene to cherish but as an opportunity for carnage," she said.
British police said an eight-year-old girl, Saffie Roussos, was among the dead and that 12 girls under the age of 16 were among the injured.
May said authorities "now need to know whether the bomber was acting alone or is part of a wider group."
"It will take some time to establish these facts, and the investigation will continue," May said.
Sky TV quoted police as saying the bomber was killed when he detonated his explosives.
Greater Manchester Police's Chief Constable Ian Hopkins said on May 23 that the suspect bomber, Abedi, was 22, but gave no further details.
"Priority is to continue to investigate whether he was acting alone or working as part of a wider network," Hopkins added.
The Daily Telegraph newspaper reported that Abedi was born in Manchester in 1994 and that his parents had come to England as refugees from Libya looking to escape the regime of Muammar Qaddafi.
Witnesses at the 20,000-capacity Manchester Arena said they heard a "massive explosion" at around 10:35 p.m. local time on May 22, moments after U.S. singer Ariana Grande had finished performing.
IS claimed responsibility in a statement published on its social-media channels. The statement said that "one of the caliphate's soldiers placed bombs among the crowds," and threatened more attacks.
But the top U.S. intelligence official told a hearing of the Senate Armed Services Committee on May 23 that the United States has not confirmed the militant group’s claim.
Dan Coats, the U.S. Director of National Intelligence, said the group "has claimed responsibility for the attack in Manchester although they claim responsibility for virtually every attack. We have not verified yet the connection."
WATCH: Concert Blast Witness: 'There Was Pandemonium Everywhere'
Leeds resident Gary Walker told the BBC he saw the bomber make his way into the entrance foyer of Manchester Arena as the crowd began leaving.
Walker, who was waiting by the box-office entrance to pick up his daughter, said the bomber carried his explosives with him and detonated the device in an area where concert merchandise was being sold.
Other eyewitnesses reported seeing metal nuts and bolts spread across the floor after the blast along with the bodies of the dead -- an indication that whoever built the explosive device wanted shrapnel to kill or maim as many people as possible.
WATCH: Crowds Flee After Concert Blast In Manchester
Home Secretary Amber Rudd called the bombing a "barbaric attack, deliberately targeting some of the most vulnerable in our society -- young people and children out at a pop concert."
In the aftermath of the blast, British political parties agreed to suspend all national campaigning for Britain’s June 8 parliamentary elections until further notice.
Meanwhile, political leaders around the world condemned the attack.
U.S. President Donald Trump on May 23 the attack was carried out by "evil losers."
Speaking alongside Palestinian Authority President Mahmud Abbas after talks in the West Bank city of Bethlehem, Trump said the attack preyed on "innocent children."
Trump also said "civilized nations must join together to protect human life" and this "wicked ideology must be obliterated. And I mean completely obliterated."
WATCH: Trump Offers Condolences To U.K. Victims, Calls Attackers 'Evil Losers'
German Chancellor Angela Merkel voiced "sorrow and horror" over the attack, saying on May 23 that Germany would stand by Britain in the fight against terror.
"This suspected terrorist attack will only strengthen our resolve to work with our British friends against those who plan and execute such inhuman acts. I assure the people in Britain: Germany stands by your side," she said in a statement.
French President Emmanuel Macron also expressed "horror" at the attack, some of whose victims were children, and said he would speak to Theresa May.
European Commission President Jean-Claude Juncker expressed "great sadness and profound shock" over the "brutal attack."
"It breaks my heart to think that, once again, terrorism has sought to instill fear where there should be joy, to sow division where young people and families should be coming together in celebration," he said.
A Kremlin statement quoted Russian President Vladimir Putin as saying his country firmly condemns "this cynical, inhuman crime."
Chinese state broadcaster CCTV reported that Chinese President Xi Jinping sent a message to Queen Elizabeth expressing his "sincere condolences."
"The Chinese people are firmly standing together with the British people at this difficult time," the channel quoted Xi as saying.
Britain's Queen Elizabeth II sent her sympathy to the victims of the attack. She also expressed “admiration for the way the people of Manchester have responded, with humanity and compassion, to this act of barbarity."
Authorities across Britain were on heightened alert following the attack, and many Britons were on edge.
Shortly before noon on May 23, police evacuated the Arndale Shopping Center in central Manchester after witnesses said they heard a "big bang."
The shopping center was reopened within an hour after police arrested one man there. Authorities said the arrested man was not thought to have been linked to the May 22 terrorist attack.
Meanwhile, London's busy Victoria Coach Station was closed for several hours early on May 23 while authorities investigated a report of a "suspect package" at the major bus depot.
Later, London's Metropolitan Police said they had determined that the package was "not suspicious."
The attack also set off a nightlong search for loved ones in Manchester after thousands of panicked children and teenagers fled the arena, making it difficult for desperate parents to find them.
With public transport shut down, taxis offered to give free rides to stranded people and some residents opened their homes to provide lodging.
Concertgoers who had parked their vehicles at the arena were still being prevented from entering the facility's parking areas during the morning of May 23, leaving hundreds of out-of-town visitors stranded.
Grande, 23, posted a tweet several hours after the blast saying, "Broken. From the bottom of my heart, I am so so sorry. I don't have words."
Grande also indefinitely suspended the 35 remaining concerts of her Dangerous Woman Tour, a world tour that was scheduled to continue through September 21 with concerts across Europe, Latin America, Asia, and Australia.
Media reports quoted sources in Grande's entourage as saying her team will reevaluate the touring schedule when she is emotionally ready to do so.