Manchester - Britain raced on Wednesday to track down a jihadist network suspected to have orchestrated the Manchester concert bombing, as soldiers fanned out to guard key sites under a maximum terror alert.
Investigators were trying to piece together the last movements of suicide bomber Salman Abedi, a Manchester-born university dropout whose parents had reportedly fled the now fallen regime of Libyan dictator Muammar Gadaffi.
A French minister said he may have been radicalised in Syria.
"It's very clear that this is a network that we are investigating," Manchester police chief Ian Hopkins told reporters, with four people under arrest after a pair of armed raids on addresses in the city.
Monday night's bombing, which killed 22 people including a girl aged just eight, was claimed by the Islamic State group which warned of more attacks to come.
It was the latest in a series of deadly incidents across Europe claimed by ISIS jihadists that have coincided with an offensive on the group's redoubts in Syria and Iraq carried out by US, British and other Western forces.
Officials said the 22-year-old Abedi had been on the radar of the intelligence community before the massacre at a concert by US pop star Ariana Grande, and warned another attack "may be imminent".
After arresting a 23-year-old man on Tuesday, police said they had taken three more men into custody on Wednesday in south Manchester, where Abedi lived.
An armed raid was also carried out in Manchester city centre on Wednesday, during which police said a nearby railway line had to be "briefly closed".
Hundreds of armed military personnel meanwhile fanned out to take up guard duties at the British parliament and Buckingham Palace - a highly unusual sight on the streets of Britain since the end of the Northern Ireland conflict in the 1990s.
French Interior Minister Gerard Collomb said Abedi had "likely" been to Syria, citing information provided by British intelligence services to their counterparts in Paris.
Collomb told French television the suspect "grew up in Britain and then suddenly, after a trip to Libya and then likely to Syria, became radicalised and decided to carry out this attack".
"In any case, the links with Daesh are proven," he said, using a term for the Islamic State group.
In light of the Manchester attack, Nato chief Jens Stoltenberg said the military alliance had to "step up and agree to do more in the fight against terrorism" at summit talks set for Thursday.
The summit is to be joined by US President Donald Trump, who has lambasted Nato for not doing more against Islamist extremism and who called those responsible for the Manchester bombing "evil losers".
British Prime Minister Theresa May placed the country on its highest level of terror alert - "critical" - for the first time since June 2007, when it was sparked by an attack on Glasgow airport.
The Changing of the Guard, a military ceremony in front of Buckingham Palace popular with tourists, was cancelled on Wednesday and the Houses of Parliament suspended all public events.
Chelsea football club said they were cancelling their Premier League victory parade on Sunday saying it would be "inappropriate".
Manchester United players will wear black armbands at Wednesday evening's Europa League final against Amsterdam club Ajax in Stockholm, where a minute's silence will be held.
The attack was the deadliest in Britain since July 7, 2005 when four suicide bombers inspired by al-Qaeda attacked London's transport system during rush hour, killing 52 people.
A Polish couple living in Britain were confirmed among the Manchester victims, along with 15-year-old Olivia Campbell, whose mother had issued heartrending appeals for help when her daughter was still missing.
Next to a photograph of Olivia, Charlotte Campbell wrote on Facebook: "RIP my darling precious gorgeous girl Olivia Campbell taken far far too soon. Go sing with the angels and keep smiling, mummy loves you so much."
A total of 64 people are being treated in hospital, including 20 in critical care, medical officials said.
Twelve of the injured are aged under 16.
Nick Lewis said his daughter Freya had been through 10-and-a-half hours of surgery.
"Freya has been sewn, bolted, drilled and bandaged back together. It is going to be a long climb, but we are on the first step," he said.
The plan for this type of troop deployment, code named Operation Temperer, was first revealed after the November 2015 Paris terror attacks and is believed to allow up to 5 000 troops to be deployed.
The last time troops were deployed on British streets was after a suspected airliner plot in 2003.
France has already grown used to the sight of rifle-toting soldiers in its major cities under a state of emergency imposed after the attacks on the Bataclan concert hall and elsewhere in Paris.
Earlier on Wednesday, French President Emmanuel Macron said he would seek an extension to the state of emergency until November.