Prime Minister Theresa May said the operation had been authorised by Defence Secretary Sir Michael Fallon at the request of the police after experts at the Joint Terrorism Analysis Centre (JTAC) raised the threat level from "severe" to the highest "critical" setting.
"You might also see military personnel deployed at certain events such as concerts and sports matches, helping the police to keep the public safe".
The country's senior anti-terror police officer said there were "gaps in our knowledge" about Abedi which had led to the increased threat level.
France's interior minister said Abedi is believed to have traveled to Syria and had "proven" links with the Islamic State group.
German Chancellor Angela Merkel said it "will only strengthen our resolve to. work with our British friends against those who plan and carry out such inhumane deeds".
After years of successfully fending off more sophisticated strikes even as countries across continental Europe have fallen victim to bombings, Monday night's carnage reflected the fact that Britain is hardly immune amid a rising tide of extremist violence.
The Manchester attack was the deadliest in Britain since July 2005, when four British Muslim suicide bombers killed 52 people in coordinated attacks on London's transport network.
Witnesses described the horror when the bomber blew himself up.
"We were all praying for her to be found safely", Shelby Wharton, a 17-year-old friend, told the Evening Standard.
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Grande had not been seen publicly since an explosion ripped through the packed Manchester Arena at the end of her performance there.
The disclosure came as Home Secretary Amber Rudd confirmed the United Kingdom security services had been aware of the British-born 22-year-old.
Police promised extra measures at showpiece events coming up such as Saturday's FA Cup football final.
"The Labour Party's General Election campaign remains suspended until further notice", a party spokesman said.
A 23-year-old man has been arrested in connection with the investigation.
President Donald Trump reacted to the bombing while on a trip in the Middle East, calling those behind it "evil losers". Thoughts and prayers with all your family and the other 21 people who lost there lives last night'.
That attack was also claimed by IS, as was one in March by a knifeman at the gates of the British parliament-although police downplayed that claim.
At Buckingham Palace, Queen Elizabeth II marked a moment of silence alongside her husband Prince Philip as well as Prince Charles and his wife Camilla. So many people had been following the story as Olivia became the face of the tragedy as photos of the pretty teen circulated all over social media.
Gerolymatos agrees; people who commit acts of terror, particularly suicide attacks, don't typically blend in with the crowd.