Britain ordered soldiers to key sites on Wednesday after going on maximum alert for fear of an imminent new terror strike following a suicide bombing at a pop concert in Manchester.
Security services believe the suspected bomber, Salman Abedi, was likely to have had help from others in staging the massacre that killed 22 people including one girl aged just eight.
Interior minister Amber Rudd said the 22-year-old had been on the radar of the intelligence community before the attack late Monday at a concert by US pop star Ariana Grande.
Security services were trying to piece together the last movements of Abedi, a Briton of Libyan descent, amid media reports he had travelled recently to Libya and possibly Syria.
Rudd declined to give any further information about Abedi but told BBC radio: "It was a devastating occasion, it was more sophisticated than some of the attacks we've seen before, and it seems likely -- possible -- that he wasn't doing this on his own."
The minister said she was "not surprised at all" that the attack had been claimed by the Islamic State (IS) militant group, but said there was no information yet to confirm the extremist organisation's active direction.
Prime Minister Theresa May placed the country on its highest level of terror alert -- "critical" -- for the first time since June 2007, following an attack on Glasgow Airport.
The last time troops were deployed on British streets was after an airliner plot in 2007.
Troops will fan out at sites such as Buckingham Palace, Westminster and foreign embassies in London to free up armed police for anti-terror duties.
May said a new attack "may be imminent" but the authorities stressed that the soldiers would remain under police command.
"This is a temporary arrangement in order to respond to an exceptional event," Rudd said.
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 listed as missing.
Next to a photograph of Olivia, Charlotte Campbell 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."
A total of 59 people were taken to hospital, many with life-threatening conditions. Twelve of them were aged under 16.
British media said Abedi was born in Manchester and that his Libyan parents had fled the regime of dictator Moamer Kadhafi.
Police on Tuesday staged an armed raid on a Manchester address believed to be where Abedi lived, carrying out a controlled explosion to gain entry after arresting a 23-year-old man as part of the investigation.
Abedi was reported to be a former business student who dropped out of university and turned to radical Islam.
Britain's best-selling newspaper The Sun carried pictures Wednesday of eight-year-old Saffie Rose Roussos, who was killed in the attack, and of Abedi under the words "PURE" and "EVIL" respectively.
Monday's attack came just over two weeks before Britain votes in a general election and campaigning remained suspended by all the main political parties.
The plan for the troop deployment, which has never before been used and is codenamed 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.
France has already grown used to the sight of rifle-toting soldiers on the streets of its major cities under a state of emergency imposed after the attacks on the Bataclan concert hall and elsewhere in Paris.
British police had already announced extra security measures for upcoming sporting fixtures including Saturday's FA Cup football final.
Manchester was the latest in a series of deadly incidents across Europe claimed by IS that have coincided with an offensive on the group's redoubts in Syria and Iraq carried out by US, British and other Western forces.
In a city famed globally for its football teams and pop bands like Oasis, The Stone Roses, The Smiths and Take That, showbusiness stars and teams have joined political leaders worldwide in expressing their horror at the carnage.
"Words don't really do justice for how we all feel. We're numb," Manchester United's executive vice-chairman Ed Woodward said, ahead of Wednesday evening's Europa League final against Amsterdam giants Ajax.
The players will wear black armbands for the game in Stockholm and a minute's silence will be held.
In the city centre Tuesday evening, thousands gathered for a multi-faith vigil outside the town hall to remember the victims, with defiant chants of "Manchester! Manchester!" ringing around.