Manchester city gathered in the city’s Albert Square to remember the victims of yesterday’s bomb attack.
Britain is dealing with an unprecedented terrorist threat which has seen MI5 and police disrupt five terror plots in the past two months alone, a senior Whitehall source has said.
The threat from Islamist jihadists intent on committing attacks in the Britain is so high that the security services are currently running 500 active investigations looking at some 3000 potential suspects.
Counter terrorism officials yesterday sought to disclose the scale of the menace as MI5 and police faced accusations they had missed chances to stop the Manchester bomber when he was repeatedly flagged to authorities as a danger.
Family, friends and the local community are understood to have informed the authorities of the danger posed by Salman Abedi on at least five separate occasions before he blew himself up at a Manchester Arena pop concert on Tuesday, killing 22 people.
As the country remained on its highest terror alert for a decade, it was announced armed transport police would patrol trains for the first time.
With the public urged to be vigilant, police were called to a string of false alarms. Bomb squad officers were called to a school in Manchester, while Westminster Bridge and a shopping centre in Newport were both closed because of suspicious cars and Swansea magistrates' court was evacuated over a suspect package.
The threat from battle-hardened jihadists returning from Iraq and Syria and the peril of online radicalisation are contributing to the highest threat seen in decades.
A total of 18 plots have been uncovered since 2013, including five in the two months since Khalid Masood killed four people during a car and knife rampage in Westminster.
The source said: "Abedi was one of a larger pool of former subjects of interest whose risk remained subject to review by MI5 and its partners."
The source said where former subjects of interest seemed to show a risk of heading back into terrorism "MI5 can consider re-opening the investigation, but this process inevitably relies on difficult professional judgements based on partial information".
A terror attack in Britain is expected imminently after the threat level was raised to critical in the wake of Tuesday's attack. The question of whether counter-terrorism forces have enough funding or resources for the fight against terrorism is now likely to become a general election issue.
Charlotte Campbell and Paul Hodgson (centre), the mother and stepfather of victim Olivia Campbell, pay tribute to those killed on Tuesday. Photo / AP
One former senior security figure said: "Knowing of someone's radical sympathies and knowing they present a real and present danger are very different things.
"So the essence of the security dilemma is triage, how to assess who and when to investigate very deeply given the resources needed for 24/7 surveillance. For every suspect that appears to be high priority another has to be pushed down the list.
"So who not to investigate urgently is as important a decision as who might be worth investigating."
Shashank Joshi, senior research fellow at security think tank the Royal United Services Institute, said: "It's easy, with the benefit of hindsight, to argue that these warnings were opportunities to stop the bomber.
"However, it's also possible that these warnings were followed up, surveillance was conducted, and nothing was discovered.
"Authorities cannot keep monitoring a suspect indefinitely, given limited resources. There may, however, be questions over his travel to Libya, Germany, and perhaps Syria, and his ease of return to the UK afterwards. "It may point to weaknesses in the system of monitoring onward travel, especially as the number of UK nationals visiting Libya is likely to be fairly small."
About 1000 troops remained on the streets after the Government invoked the Operation Temperer contingency plan allowing police to call on military support.
Soldiers taking up guard duty at nuclear installations, high-profile sites and large public events are freeing up armed police to carry out counterterrorism patrols.
However David Blunkett, a former Home Secretary, said he had had reservations about authorising their use in 2003 in response to concerns about al-Qaeda. He told the BBC: "You should use military personnel very sparingly indeed in a democracy.
"If there is an insurgency and therefore you know that people are at risk then of course you would use the military but it is only in very rare circumstance that you would even backfill.
"I have no problem with the military outside Buckingham Palace. I would have considerable problems if the military were used, for example, this weekend at Wembley."
