Britain has raised its terror alert to the maximum level and ordered troops to protect strategic sites after 22 people were killed in a suicide bomb attack on a Manchester pop concert.
Here is what we know so far about Monday's attack, the deadliest in Britain since 2005.
Police said they were called at 10:33 pm Monday to reports of an explosion at Manchester Arena during a concert by US pop star Ariana Grande, who is popular with teenagers and pre-teens.
Witnesses described a "huge bomb-like bang" and scenes of panic as young fans rushed out and parents waiting outside searched frantically for their children.
The bomber used an improvised explosive device apparently packed with nails and other metal objects, outside one of the exits at the 21,000-capacity arena.
Photographs published by The New York Times on Wednesday appeared to show that the bomb packed a powerful charge together with a shrapnel of nuts and screws carefully arranged to cause maximum damage.
The images also suggest the suicide bomber — or a possible team helping him — had a remote detonator setup, to back up a hand-held detonator found at the scene.
Who is behind it?
The suspected bomber has been identified as 22-year-old Salman Abedi, a British student dropout born to Libyan parents who fled the regime of slain dictator Moamer Kadhafi.
Interior minister Amber Rudd confirmed on Wednesday that Abedi was known to intelligence services.
Manchester Chief Constable Ian Hopkins said that Abedi had not acted alone.
"It's very clear that this is a network that we are investigating," Hopkins said.
British police on Wednesday made six more arrests, including one woman, mostly in and around the Manchester area, after detaining a 23-year-old man on Tuesday.
One of the men arrested was carrying a suspect package was detained in Wigan, west of the city, while the seventh suspect was arrested further afield in Nuneaton, Central England.
And Libyan security forces arrested Wednesday a younger brother and the father of Abedi, with a family source saying that the brother, like Salman, was also born in Britain.
A police spokesman said that the brother had been aware of the attack plan, and that both men were IS members.
French interior minister Gerard Collomb said Abedi had become radicalised after a trip to Libya and probably Syria, according to information received from British intelligence services.
The Islamic State group has claimed responsibility through its social media channels, saying "one of the caliphate's soldiers placed bombs among the crowds", and threatening more attacks.
Who are the victims?
Twenty-two people were killed and 64 were injured and being treated in hospital, including twelve under the age of 16, medical officials said.
Twenty of the injured are in critical care.
Eight-year-old Saffie Rose Roussis is so far the youngest named victim. She attended the concert with her mother and older sister, who were both injured.
Olivia Campbell, 15, was confirmed dead on Wednesday by her mother, who had issued heartrending appeals for help when her daughter was still listed as missing.
A Polish couple living in Britain, identified as Angelika and Marcin Klis, were also caught in the explosion as they went to collect their daughters.
And Hopkins, the chief constable, confirmed Wednesday that a serving police officer was among the victims.
Prime Minister Theresa May on Tuesday placed the country on its highest level of terror alert — "critical" — meaning a new attack is believed to be imminent.
Soldiers are being sent to assist armed police to protect strategic sites. The last time troops were deployed on British streets was in 2007.
The troop plan, codenamed Operation Temperer, was first revealed after the November 2015 Paris attacks and is believed to allow up to 5,000 soldiers to be deployed.
Major sports venues plan to beef up security, with several high profile events in the coming weeks, including the FA Cup final at Wembley on Saturday.
The Changing of the Guard outside Buckingham Palace was cancelled on Wednesday and parliament suspended all public events.
Manchester United's Europa League final against Ajax in Stockholm went ahead Wednesday under tight security.
Downing Street said May would still attend the NATO summit in Brussels on Thursday but cut short her trip to a G7 meeting in Italy this week, attending on Friday but returning home before the summit ends on Saturday.
Other attacks on UK soil
The Manchester bombing is Britain's second terror attack in two months.
On March 22, five people were killed and more than 50 injured when a man ploughed into pedestrians on Westminster Bridge in central London, before crashing into the fence surrounding parliament.
The attacker, 52-year-old Khalid Masood, fatally stabbed a police officer before being shot dead by police outside parliament.
Investigators described that attack as "Islamist-related terrorism" but have not charged anyone over the incident.
The deadliest bomb attack on British soil took place on July 7, 2005 when four British suicide bombers inspired by Al-Qaeda attacked London's transport system, killing 52 people and wounding 700.