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Britain was placed in lockdown today as soldiers were put on guard outside Buckingham Palace and Downing Street.

As the police continued a frantic hunt for accomplices of the Manchester bomber, Scotland Yard announced that troops were being deployed to guard “key locations” including the Palace of Westminster, embassies and other sensitive sites in major cities.

The UK’s threat level has been raised to critical with 984 military personnel being deployed around the country

In London, Changing the Guard at the Palace was cancelled and Parliament was closed to the public. The Old Bailey’s public galleries were also shut.

Soldiers are likely to be on duty at this weekend’s FA Cup final at Wembley and the Twickenham rugby premiership final.

Armed police will guard concerts in the capital this weekend and at major events such as the Chelsea Flower Show.

There are fears an Islamist bombmaker may be on the loose because of the sophistication of the device used in the Manchester attack on Monday.

Security sources said police and intelligence agencies were working flat out to prevent “more bad stuff”.

Up to 3,800 soldiers were being deployed to protect sites across the UK as armed police officers mounted “roaming” patrols around train stations, shopping centres and other sites.

It is understood that 400 to 800 troops will be deployed in the first wave.

Three men were arrested in south Manchester today in connection with the attack.

The suicide bomber, Salman Abedi, 22, returned from Libya in recent days and, it is reported, may have travelled to Manchester by train from London.

It raises the possibility that the bomb was built in the capital.

Announcing the raising of the threat level last night, Theresa May said a “wider group of individuals” could have been involved in the blast, which killed 22 people, including an eight-year-old girl.

Home Secretary Amber Rudd said that Abedi had recently returned from Libya, where his parents are reported to live, and said the nature of the attack suggested he may have had support.

A total of 119 people were injured in the explosion in the foyer of the Manchester Arena, shortly after a concert by US pop star Ariana Grande.

Today, 20 people remain in “critical care” across Manchester. People have “horrific injuries” including major organ damage and lost limbs.

Fears are also growing for up to 15 people who are thought to have attended the concert but who have still not been traced.

Victims who have been named so far include 15-year-old Olivia Campbell, from Bury, Greater Manchester, eight-year-old Saffie Roussos and Georgina Callander, 18. Saffie, from Preston, is the youngest known fatality and had attended the concert with her mother Lisa and sister Ashlee Bromwich.

Chris Upton, headteacher at Tarleton Community Primary School, said: “Saffie was simply a beautiful little girl in every aspect of the word. She was loved by everyone and her warmth and kindness will be remembered fondly.”

One mother waiting to pick up her children today described witnessing the suicide bomber as he exploded his device. Emma Johnson had gone to collect her daughter, 15, and son, 17, from the concert and was waiting in the foyer when the nail bomb detonated.

She described seeing a man in an “intense” red top — who she now understands to have been Abedi — among a sea of teenagers.

She told BBC Radio 5 Live: “I was about 15 feet away from the blast. We were waiting for our children to come out and we stood at the top of the foyer — you go up some stairs and we were protected by glass on a barrier.

"As people were coming out they were wearing the clothes of Ariana — you know the white, the black, the pink, because they all sold the merchandise.

"But for one split second I turned and saw what I can only describe as a bright red — that’s why it stood out — bright red, with a grey panel down the front with risen bits all over it.

“It was that that stood out because it was so intense, the colour, in this crowd of people. As quick as I saw it the explosion happened.”

Ms Rudd described the attack as “more sophisticated than some of the attacks we have seen before”. She said: “It seems likely — possible — that he was not doing this on his own.”

She said the security services knew Abedi “up to a point” — and promised that details of what was known would be revealed after the investigation.

Ms Rudd said the military deployment, Operation Temperer, would provide up to 3,800 troops to support the police. Speaking this morning, she said the move was temporary but refused to be drawn on how long the heightened state of alert would remain in place.

Detectives are today questioning Abedi’s 23-year-old brother Ismail, who was arrested in Manchester yesterday. Further arrests are expected today.

Security services are investigating whether Salman Abedi had received training abroad. Born in Manchester, he was the second youngest of four children whose parents were Libyan refugees who fled to the UK to escape the regime of Muammar Gaddafi.

Abedi’s parents have since returned to Libya he is thought to have made several trips there in recent years. One friend said he travelled there three weeks ago and returned days ago.

Today, France’s interior minister also revealed that the bomber is believed to have travelled to Syria and had “proven” links with Islamic State.

Gerard Collomb told French TV that both British and French intelligence services had information that Abedi had been in Syria.

He said : “All of a sudden he travelled to Libya and then most likely to Syria, became radicalised and decided to commit this attack.”

Britain’s senior anti-terror police officer said there were “gaps in our knowledge” about Abedi, which had led to the threat level being raised to its highest in 10 years.

Met Assistant Commissioner Mark Rowley said: “We are moving at pace, we are making arrests, we are doing searches but, not unsurprisingly, there are still gaps in our knowledge.”

US officials say Abedi’s family had warned the British authorities he was dangerous before the attack, he had ties to al Qaeda and had received terror training abroad.

Abedi was identified by a bank card found in his pocket at the scene of the explosion, the official said.

The US official told MSN that Abedi’s bomb was ‘big and sophisticated,’ using materials hard to obtain in Britain. ’It’s almost impossible to see he didn’t have help,’ he added.

Detectives are also unravelling the bomber’s links to extremists in this country in an effort to establish how and when he was radicalised.

In London armed officers were deployed outside train stations this morning in a visible sign of the increased threat level.

A unit of officers brandishing machine guns was on guard outside Victoria station while inside an armed patrol was on the concourse.

There was also a noticeable increase in police activity around Parliament and Downing Street and along Whitehall as security was visibly tightened.

Commuters and tourists at Victoria station said they believed the extra security was necessary after the Manchester bombing.

Guy Hammond, 36, said: “It helps to reassure people in these times. It’s sad it’s come to this though.”