Almost 1,000 troops have been deployed to help the police guard "key locations" initially in London but then elsewhere, the Home Secretary has said.
The 984 are part of the 3,800 soldiers who will assist armed police under Operation Temperer, which has been enacted after warnings that another terrorist attack could be imminent.
There will also be an increase in police numbers and operations with immediate effect.
During this temporary deployment, soldiers will guard major sites such as rail stations, Parliament, embassies and Buckingham Palace.
Soldiers will also arrive at Sellafield nuclear plant, Cumbria, on Wednesday to provide additional security.
Extra armed officers will provide support for Project Servator, an operation which sees undercover officers trying to spot people carrying out "hostile reconnaissance" and other criminal activity.
In a live televised statement on Tuesday evening, Prime Minister Theresa May said the terror threat level would be increased to "critical", meaning an attack was expected imminently.
"This means that armed police officers responsible for duties such as guarding key sites will be replaced by members of the armed forces, which will allow the police to significantly increase the number of armed officers on patrol in key locations.
"You might also see military personnel deployed at certain events such as concerts and sports matches, helping the police to keep the public safe."
This is because a "wider group of individuals" could have been involved in the Manchester Arena attack rather than just suicide bomber Salman Abedi.
Monday night's attack at a concert by US pop star Ariana Grande left 22 people dead, including an eight-year-old girl, and dozens injured.
Speaking at Number 10, Mrs May said:
"The spirit of Manchester and the spirit of Britain is far mightier than the sick plots of depraved terrorists.
"That is why the terrorists will never win and we will prevail."
Mrs May will chair a meeting of the Government's emergency Cobra committee at 9.30am on Wednesday, Downing Street has announced.
The suicide bomber has been named as Salman Abedi, a 22-year-old man believed to have been born in Manchester and of Libyan descent.
Abedi studied business at Salford University, but dropped out before completing his degree.
He is thought to have attended the Manchester Islamic Centre, also known as Didsbury Mosque, along with his parents and siblings.
A family friend, who asked not to be named, described him as "normal" and said they were known to the Libyan community in the city.
He told the Press Association: "He was always friendly, nothing to suggest (he was violent). He was normal, to be honest."
Abedi was named after armed officers carried out a raid on the red brick semi in south Manchester where he was registered as living.
The terror threat level was increased after investigations revealed Abedi may not have acted alone.
Mrs May said:
"The work undertaken throughout the day has revealed that it is a possibility we cannot ignore that there is a wider group of individuals linked to this attack.
"This morning I said that the Joint Terrorism Analysis Centre, the independent organisation responsible for setting the threat level on the basis of the intelligence available, was keeping the threat level under constant review.
"It has now concluded, on the basis of today's investigations, that the threat level should be increased for the time being from severe to critical.
"This means that their assessment is not only that an attack remains highly likely, but that a further attack may be imminent."