The threat level in London remains at "severe" and the increase is not in response to any specific intelligence but is a long-term response to a spate of deadly attacks in European countries.
"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," Metropolitan Police Commissioner Bernard Hogan-Howe said in a statement, Reuters reported.
In line with British policing methods, the majority of London police officers do not carry firearms.
"The reality of having to deal with armed and deadly attackers is that you need firearms officers who will use force to stop those attackers in their aim. Our firearms officers are the ones who will run towards the danger," said Hogan-Howe.
The additional armed officers will be visible to the public and will work across London, both on foot and in vehicles.