As part of a new anti-terrorism operation, 600 additional armed officers will be present in highly visible locations across the UK capital to protect against the threat of attacks.
In a statement, Metropolitan Police Commissioner Bernard Hogan-Howe said: "Anyone who's been following events in Europe over the past few weeks will understand why we want to show our determination to protect the public."
He said authorities were "deadly serious" about protecting the people of London and that the armed officers were there to reassure and help the public against any attackers.
"Our firearms officers are the ones who will run toward the danger," he said.
While the threat level has not changed and remains at "severe," authorities said it's important they're prepared following recent attacks in France
London Mayor Sadiq Khan said that while additional armed police will be on the streets, there is no reason to be alarmed.
"It is important we are prepared should the unthinkable happen," Khan said.
While the operation is not based on any specific intelligence, Hogan-Howe said earlier this week that the risk of a terror attack in the UK is a case of "when, not if."
He said on the police's official blog
Sunday he believes the threat of terrorism is real and expresses "a greater sense of fear that Britain will be the next victim."
Britain's senior counterterrorism officer, Mark Rowley, also has admitted police were preparing for "enormous and spectacular" attacks
on the UK. A police press officer later said Rowley's comments did not refer to a specific known plot but a general threat.
Borders and coastal security
Meanwhile, a new report from Britain's Home Affairs Committee
said its Border Force has a "worrying low" number of boats patrolling the UK coastline.
It said only three boats were available to patrol more than 7,500 miles of coastline, compared with some other European countries that have at least 100 vessels for much smaller coastlines.
The Border Force "has been given a key role in implementing strengthened coast security measures," the report said. But it is under-resourced due to "problems in gaining access to a sufficient number of patrol boats."
The committee said rigorous steps needed to take place, as smaller ports are being used by criminal gangs to move people between Europe and the UK.
The force said it needed to be given all the necessary equipment, including vessels, to allow it to carry out its responsibilities effectively.
"Royal Navy vessels should be made available to Border Force to make up for shortfalls, where necessary."