British Prime Minister Theresa May has announced that the terror threat level in the UK has been raised to 'critical' - the highest level - following the Manchester attack.
An 8-year-old girl is believed to be among the 22 people killed in the suspected suicide bombing at the city's Manchester Arena last night.
59 others are being treated in hospitals across the city.
The raising of the threat level from 'severe' to 'critical' indicates concerns that another attack could be imminent.
Mrs May announced that the number of armed officers will be increased in 'key locations' and 'certain events' around the country, but insisted she did not want the public 'to feel unduly alarmed'.
Speaking an emergency meeting this evening, she said: "It is a possibility we cannot ignore that there is a wider group of individuals linked to this attack."
The suspect behind last night's terror attack in Manchester has been named as 22-year-old Salman Abedi.
Mrs May said Abedi "was born and brought up" in Britain.
Earlier, the Islamic State group said one of its members carried out the attack, which happened after an Ariana Grande concert at the venue.
In a statement on social media, the terror group said one of its soldiers had 'placed bombs among the crowds'.
Manchester Police named the suspect this evening, but noted that he has not yet been formally named by the coroner.
A 23-year-old man has also been arrested in connection with the bomb attack.
Latest statement from @ccianhopkins in relation to last night's attack. pic.twitter.com/w1BdLp9nY4
— G M Police (@gmpolice) May 23, 2017
A number of children have been confirmed dead following the bombing.
Several victims have been identified as those killed at the concert.
Meanwhile, thousands attended a vigil in Manchester this evening following last night's tragedy.
"It will not succeed"
The British Home Secretary Amber Rudd confirmed the blast - which is believed to have been caused by an improvised explosive device - was a terrorist attack.
Ms Rudd said the attack was an attempt to "sow fear" and "divide" - but insisted it will not succeed.
"The great city of Manchester has been affected by terrorism before. Its spirit was not bowed, its community continued," she said.
"This time it was a particular attack on the most vulnerable in our society. Its intention was to sow fear, its intention is to divide - but it will not succeed."
In Dublin, Foreign Affairs Minister Charlie Flanagan said the attack was a "deliberate, callous, targeting of young children and teenagers" adding that there is no evidence of any Irish citizens caught up in the attack:
WATCH: Minister @CharlieFlanagan reacts to #Manchester attack. Says no evidence of any Irish caught up in it pic.twitter.com/8Mpe4LgPsc
— Sean Defoe (@SeanDefoe) May 23, 2017
Several young people have been reported missing following the explosion - with concerned friends and relatives appealing for information on Twitter.
A hotline has been set up for those with concerns over loved ones who remain unaccounted for.
Anyone with concerns for Irish citizens in Manchester can contact the Irish embassy in London on +44-207-2358-2171 or the Department of Foreign Affairs in Dublin on 01-408-2000.
Irish reaction
In a statement, the Taoiseach Enda Kenny said: "The vile acts carried out in Manchester last night are a reminder of the depravity of the views held by the few."
"Those beliefs have no place in our society," he said.
"My thoughts and prayers are with the victims, their families and all those affected by this atrocity. I am heartbroken for all.
"The city of Manchester has exceptionally close ties with our country and I extend the solidarity of the Irish Government and all our people to those affected across the UK."
Earlier, he tweeted his condolences to victims of the attack and their families:
Our thoughts & prayers are with the victims, their families & all those affected by the atrocity in Manchester. We are heartbroken for all.
— Enda Kenny (@EndaKennyTD) May 23, 2017
Fianna Fáil leader Micheál Martin has offered his sympathies to all those affected by “last night’s heinous attack in Manchester".
"To attack a group of young people in such a cowardly way is simply abhorrent to the vast majority of people in Ireland, and I know that the people of Ireland stand in solidarity with the people of Manchester,” he said.
"As a parent, I find this attack exceptionally difficult to comprehend, and while details are limited at present, it's clear that whoever orchestrated this attack cares little for basic decency and humanity.”
Our thoughts and prayers are with all the families who have lost loved ones as a result of this barbaric act in Manchester.
— Micheál Martin (@MichealMartinTD) May 23, 2017
Independents 4 Change TD Clare Daly became emotional in the Dáil when offering sympathy to the people of Manchester.
"Huge bang"
Witnesses reported hearing a "huge bang" at the 21,000-capacity venue at the end of the sell-out gig.
Concert-goers affected by the attack were offered shelter by residents, while taxi drivers offered free rides.
Armed police filled the streets outside the arena, and bomb disposal units, fire-fighters and paramedics also attended the scene.
Terrifying scenes
Videos from inside the venue show fans running and screaming, while footage from outside captured emergency vehicles rushing to the scene.
Officers carried out a controlled explosion at nearby Cathedral Gardens shortly after 1.30am, but confirmed the item they found was abandoned clothing and not suspicious.
Video shared on social media showed panicked crowds attempting to leave the arena:
EXPLOSION AT MANCHESTER ARENA AND EVERYONE RAN OUT SO SCARY😠pic.twitter.com/pJbUBoELtE
— ♡♡ (@hannawwh) May 22, 2017
The below footage shows the crowds reacting after the explosion was first heard:
The moment of the explosion inside the #Manchester Arena. 22 dead and more than 60 wounded, the provisional toll of the massacre. pic.twitter.com/jFSmYdIokH
— Wcn Conflict News (@NewsWcn) May 23, 2017
Grande, who was unharmed, tweeted to say she is "broken".
"From the bottom of my heart, I am so, so sorry," she said. "I don't have words."
Her manager Scooter Braun said they mourned "the lives of children and loved ones taken by this cowardly act".
In a statement, Manchester Arena said the incident had happened outside the venue "in a public space."
The US Department of Homeland Security said it was closely monitoring the situation and stands ready "to assist our friends and allies in the UK in all ways necessary as they investigate and recover from this incident."
DHS Statement on Incident at Manchester Arena → https://t.co/GOnxIvEf2p pic.twitter.com/B9NlUipLfD
— Homeland Security (@DHSgov) May 23, 2017
The explosion is the worst atrocity in the UK since the deaths of the 56 people killed in the 7/7 London bombings in 2005.
Reporting: Mick Staines, Jack Quann, Stephen McNeice
