The military will guard key London sites including Buckingham Palace and Downing Street as part of increased security measures to respond to the UK's terror threat being raised to "critical".
The Metropolitan Police Service (MPS) said the measure will "free up armed officers to carry out patrols" with military personnel also stationed at the Palace of Westminster and embassies, among other locations.
On Tuesday night (May 23) Prime Minister Theresa May announced that the terror threat had been raised, meaning that further attacks may be imminent.
Mrs May said a "wider group of individuals" could have been involved in the attack rather than just suicide bomber Salman Abedi.
As part of the new security measures, the MPS say more armed officers will be deployed in the city and will perform "proactive operations and targeted patrols, both on foot and in vehicles."
The locations, tactics and numbers will be varied continually "to be the most effective and avoid predictability", police say.
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 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.
"At this time we need the people of London to remain calm but alert.
"We need your help to help us protect the capital and thwart the terrorists who seek to divide us and undermine our way of living."
The mayor of London Sadiq Khan also made a statement this morning.
He added: "I would urge all Londoners to remain calm and vigilant.
"If you suspect something, not matter how small, report it."
Anyone concerned about suspicious behaviour, or who has information they believe may help police, should call the confidential Anti-Terrorist Hotline 0800 789 321.