Pilot patrols have already begun in Tottenham, Brixton and Haringey
The mayor of London has said he was "concerned" to learn armed police are to start patrolling gun-crime hotspots.
Officers from the CO19 unit have begun pilot patrols in Tottenham - where three Turkish men died in a drug war - as well as Brixton and Haringey.
Metropolitan Police said it was a temporary response to a 17% rise in gun-crime over the past six months.
A spokeswoman for Boris Johnson said his initial worries had since been allayed by the Chief Commissioner.
She said: "The mayor was concerned by reports about the use of armed officers but has been reassured by Sir Paul Stephenson there is no intention of using armed police in routine manner.
"Armed police have a role in certain circumstances but that should be the exception not the norm."
Other politicians opposed the approach.
Jenny Jones, a Green Party member of the Metropolitan Police Authority (MPA), said: "This is a totally unacceptable departure from normal policing tactics.
"I hope the Met will rethink this terrible decision immediately and think of a genuinely proactive way to prevent gun crime.
"I can't believe that the sight of a policeman with a machine gun will make people feel safer.
"Are we heading down a slippery slope towards armed rather than community policing?"
It is one of the first times armed police have carried out patrols in London, apart from outside high security targets such as the Houses of Parliament.
Armed police have previously carried out patrols in both Bristol and Nottingham during periods of gun-related crime.
Chief Inspector Neil Sharman, of CO19, said: "We have traditionally provided a response to gun crime.
"This new unit sees a move towards a more proactive approach to deal with weapons on our streets."