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The PM announced troops would be drafted in last night as she warned further terror attacks could be “imminent”.

And today, the first 984 infantrymen (of 5,000 immediately available) arrived in the capital as they prepare to take up their posts at strategic locations.

The military will be providing armed guards for key locations around London, including Buckingham Palace, Downing Street, the Palace of Westminster and embassies.

Scotland Yard confirmed the drastic measures in the wake of the attack by suicide bomber Salam Abedi killing 22 at an Ariana Grande concert at the Manchester Arena.

Mrs May vowed the “terrorists will not win” as she brought into force the military plan, Operation Temperer.

(Image: LIVE)

Soldiers will replace armed police at many sites, freeing them up for patrols in key areas under the plan.

The decision was taken at a meeting of the COBRA committee and means a huge military presence could be seen at major landmarks and events.

Britain has not been on critical alert for a decade – and there are fears another bloody attack could be on its way.

(Image: GETTY) (Image: GETTY)

Scotland Yard said the military will be working under its command structure to provide "static armed guarding at key locations" which includes "Buckingham Palace, Downing Street, embassies and the Palace of Westminster.

They added: "This will free up armed officers to carry out patrols".

Commander Jane Connors, leading the London policing operation, said: "We will do all we can to protect the capital that we serve at this unsettling time.

"All our work is designed to make our city as hostile an environment as possible for terrorists to plan and operate."

The soldiers joined a heightened armed presence in other cities across the UK.

But there has been criticism that no infantrymen have been sent to Manchester – despite Monday's attack.

One person tweeted: "The government want to spread these troops around instead of just concentrating on London... what about the rest of us?? Nowhere is safe."

Another said: "Troops on the streets! only in London it seems. If they are being deployed it should be in all the major cities shouldn't it?"

And one person criticised the Prime Minister for apparently only taking action to protect the capital.

They commented: "Well done PM five thousand troops brought in to protectLondonn. Can I remind you it was Manchester were the bomb went off?"

However, it is thought soldiers will be sent to other major British cities in due course.

(Image: SWNS) (Image: GETTY)

Commander Connors said of the deployments: "The reality is that we must be prepared to be able to respond to and deal with armed and deadly attackers, so we must be in a position to respond with firearms officers who will use force to stop those attackers in their aim.

"That is why we have increased the number of firearms who are on duty, both out walking and in roaming patrols, at fixed points and carrying out a range of operations."

Met Police Assistant Commissioner Mark Rowley said the first phase of Operation Temperer would allow soldiers to replace police officers at fixed locations but after that, they could "augment our patrols" at sites such as transport hubs and major events.

He said: "There are many big events across the country over the next couple of weeks, we are going to be working really closely with the organisers to review the security, review their stewarding arrangements, review our policing arrangements and make sure that decisions are taken that events only go ahead when it's sensible and safe to do so," he said.

The plan means soldiers could be deployed to support police at major events such as the FA Cup final at Wembley on Saturday or the Champions League final in Cardiff on June 3.

ISIS terrorists and their sick supporters were seen on social media celebrating the attack after claiming responsibility for the bloodbath.

Yesterday, the PM confirmed Abedi was known to MI5 – and detectives are probing the theory he was a "mule" being used by the "attack mastermind".

Daily Star Online revealed ISIS ordered their followers to build nail bombs in the weeks leading up to the Manchester bombing.

The Prime Minister said the operation had been authorised by Defence Secretary Sir Michael Fallon.

This was done at the request of the police after experts at the Joint Terrorism Analysis Centre (JTAC) raised the threat level to its highest level.

Mrs May said: "The change in the threat level means that there will be additional resources and support made available to the police as they work to keep us all safe."

The extra armed officers are part of what is being dubbed Project Servator, which sees uniformed and undercover officers trying to spot people carrying out "hostile reconnaissance" and other criminal activity on the capital's streets.

The tactic "is based on extensive research into the psychology of criminals and what undermines their activities", Scotland Yard said.

There will also be an increase in the use of stop and search, vehicle checkpoints and number plate recognition technology.