At least 22 people, including children, have died and dozens are injured after a suicide bomb attack at Manchester Arena.
The suspect, 22-year-old Salman Abedi, is believed to have had links to so-called Islamic State.
Here's everything we know so far:
- When did the attack happen?
A suicide bomber, armed with an improvised explosive device, struck a crowd at the Manchester Arena at the end of a gig by US pop star Ariana Grande.
Manchester Arena said the explosion happened outside the venue, as people began leaving the building.
- Video courtesy of Ellie Cheetham
Mobile phone footage showed the moment an explosion was heard inside the Arena with concert-goers heard screaming "what's going on?" as they began to flee the scene.
Greater Manchester Police said they were notified of the incident at 10.33pm and closed approach roads.
- What are the latest casualty figures?
Greater Manchester Police said 22 people have died and 64 people have been injured.
Those injured, 12 of whom are under the age of 16, are being treated at eight hospitals across Greater Manchester - with a number in critical conditions.
It is the worst attack in the UK since 56 people were killed in the 7/7 London bombings in 2005.
- Who are the victims?
Lancashire student Georgina Callander, 18, primary school pupil Saffie Rose Roussos, eight, and 26-year-old John Atkinson were the first people to be confirmed as victims.
Ms Callander's college reported "with enormous sadness" the death of the teenager, who had tweeted on Monday she was "so excited" to see Ariana Grande.
Saffie's headteacher spoke of her "heartbreaking" death, adding: "Saffie was simply a beautiful little girl in every aspect of the word. She was loved by everyone and her warmth and kindness."
Mr Atkinson was described as an "amazing young man" and a "true gentleman" in tributes.
Olivia Campbell, 15, whose mum Charlotte made an emotional plea for her daughter's return, also died in the attack.
She wrote 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."
- Who carried out the attack?
The suspected bomber has been identified by officials as 22-year-old Salman Abedi.
He is of Libyan descent but born in Manchester - his refugee parents having fled the regime of dictator Muammar Gaddafi prior to his birth.
Abedi visited his country of origin many times, returning as recently as this week from his latest trip.
He is thought to have been radicalised in recent months.
So-called Islamic State have claimed to be behind the terror attack.
- What was the police response?
More than 400 officers were deployed throughout Monday night while Manchester Victoria station was evacuated and trains cancelled.
Police confirmed a controlled explosion took place shortly after 1.30am at the Cathedral Gardens area near Manchester Arena, but said the suspicious item turned out to be abandoned clothing.
Armed police on Tuesday executed search warrants in two areas in the south of Manchester and carried out a controlled explosion.
Forensic officers were also seen entering a property Elsmore Road, Fallowfield.
- Have any arrests been made?
Greater Manchester Police have arrested four people since Monday evening.
- What about the terror threat level?
The UK's terror threat level was raised to critical on Tuesday - the highest it can possibly be - meaning an attack could be imminent.
Prime Minister Theresa May said the country could not ignore the possibility "a wider group of individuals linked to this attack" existed.
Scotland Yard has also announced that a number of military personnel will be deployed across London in the wake of the attack.
Armed guards from the British army are set to patrol key locations in the capital, including Buckingham Palace, Downing Street and the Palace of Westminster, as part of Operation Temperer.
- Have any events been cancelled?
In the wake of the increased terror threat, the Palace of Westminster, or the Houses of Parliament, have been closed to the public.
All public tours of parliament have been cancelled with immediate effect and any pre-planned events and banquets will no longer go ahead.
The Changing the Guard ceremony at Buckingham Palace has also been cancelled as police officers are redeployed in the wake of the attack.
- What help is available to those seeking loved ones?
Concerned relatives and friends have been urged to call 0161 856 9400 - an emergency police hotline set up after the explosion at Manchester Arena on Monday evening.
Greater Manchester Police are also offering help or assistance at the city's Etihad Stadium.
Friends and family have been mounting urgent appeals on social media for news of people missing using the hashtag #MissingInManchester.
- How can you pass on information to the police?
Anyone with information about the blast should call the national anti-terrorist hotline on 0800 789 321.
Anyone with footage from Manchester Arena that could assist the police investigation has been urged to upload it to the site for Police Appeal For Photos/Videos.
- What has been the response to the attack?
The Queen said the whole nation was shocked by the "act of barbarity".
In a statement offering her deepest sympathy to those affected, she also expressed her admiration for the way the people of Manchester have shown "humanity and compassion" in response to the "act of barbarity".
Mrs May condemned one of the "worst terrorist incidents" the UK has ever seen after chairing an emergency Cobra meeting at Downing Street.
"We struggle to comprehend the warped and twisted mind that sees a room packed with young people not as a scene to cherish but as a scene for carnage," she said.
"The people of Manchester and the UK have fallen victim to a callous terrorist attack - an attack that targeted some of the youngest people in our society with callous calculation."
All major political leaders condemned the attack and praised the response of emergency workers.
Mayor of Manchester Andy Burnham paid tribute to "the people of Manchester" as he condemned the "evil act" at the Arena.
"Even in the minutes after the attack, they opened their doors to strangers and drove them away from danger," he said of the public reaction.
"They gave the best possible immediate response to those who seek to divide us and it will be that spirit of Manchester that will prevail and hold us together."
- How did the city residents help those caught up in the attack?
City residents took to social media offering to take in any people left stranded after the explosion was reported.
Two hotels in the area - Holiday Inn and Premier Inn - also reportedly took in dozens of children who became separated from family and friends with the hashtag #RoomForManchester being widely shared.
Taxi drivers also reportedly switched off their meters to take people home
- What was Ariana Grande's response?
Pop star Ms Grande, who was not injured, said she is "broken" after the attack on her fans.
The 23-year-old wrote on Twitter: "Broken. From the bottom of my heart, I am so so sorry. I don't have words".
London's O2 Arena has indicated two of Ms Grande's shows due to take place later this week may be cancelled or postponed after the deadly attack.
- What was the international reaction to the attack?
US President Donald Trump condemned the people who commit terror atrocities as "evil losers" as he offered his "deepest condolences" during an official visit to Bethlehem.
"So many young beautiful innocent people living and enjoying their lives murdered by evil losers in life," he said.
Newly-elected French president Emmanuel Macron said his "thoughts are with the British people" while European Commission President Jean Claude Junker sent condolences to the prime minister for the "brutal attack".
German Chancellor Angela Merkel was also among the world leaders offering their condolences and expressing a determination to fight acts of terror in the wake of the attack.