After a suicide bombing at Manchester Arena on Monday left 22 dead and almost 60 injured, the United Kingdom has raised its terror threat level from severe to "critical", the highest possible level, Prime Minister Theresa May announced in a statement.
Britain's official terror threat level was raised to "critical", the highest level, late on Tuesday, meaning an attack was expected imminently.
Her dramatic statement came after 22-year-old Salman Abedi, reportedly a Briton of Libyan descent, was identified as the perpetrator behind Monday's deadly attack, whose victims included an eight-year-old girl. "And as I say, we will find out more when the operation is complete", she said.
The attack left at least 22 dead, including an 8-year-old girl, shattering the revelry at a show by American singer Ariana Grande, where strains of electric pop and the sways of innocent young fans quickly gave way to an explosion, a flood of screams and a stampede of panicked concert-goers, many clutching pink balloons and wearing the kitten-ear headbands popularized by Grande.
Rudd said up to 3,800 soldiers could be deployed on Britain's streets, taking on guard duties to free up police to focus on patrols and investigation.
May said the public "might also see military personnel deployed at certain events such as concerts and sports matches".
Started in 2006, the terror threat level is set by a unit based in the MI5 domestic security service. "This means that their assessment is not only that an attack remains highly likely but that a further attack may be imminent". Abedi, a 22-year-old British citizen born to Libyan parents, died in the attack.
"I spoke to [Salman Abedi] about five days ago. there was nothing wrong, everything was normal", Ramadan Abedi told Reuters, moments before he was arrested.
Warning of 'imminent' attack, Britain raises threat level
The sweeping investigation has caused friction between USA and British security and intelligence officials. All campaigning was suspended after the attack, although major parties said they would resume some activities on Thursday and national-level campaigning on Friday. Another 23-year-old man was arrested on Tuesday.
Witnesses described the horror when the bomber blew himself up.
He told ITV News: "It's just instinct to go and help if someone needs your help and it was children". More than 200 people were injured, although no one was killed.
Police raided two sites in the northern English city, setting off a controlled explosion in one, and arresting a 23-year-old man in a third location.
"We just held her legs up because we thought she was just going to bleed right out".
On Tuesday evening, hundreds attended a vigil outside Manchester City Hall in honor of the victims. More than 20,000 people - many of them teenagers and children - were at the sold-out concert.
Grande, who was not injured in the blast, tweeted: "broken. from the bottom of my heart, i am so so sorry. i don't have words".
US President Donald Trump and European leaders issued vows of defiance. Four men in total have been arrested in connection with the attack. Nearly 90 people were killed by gunmen inspired by Islamic State at the Bataclan concert hall in Paris during a performance by Eagles of Death Metal in November 2015.