Recent figures show that in half of all street robberies in London, a mobile phone is stolen.
Worse, people aged 14 to 17 are most likely to be the victims.
As part of the campaign, police officers will visit schools to give out safety tips, and mark phones with invisible pens.
Superintendant Archie Torrance of London's Westminster police said: "Children should be cautious when using their phones in the street.
"Be aware of who's around, especially on busy streets and outside train stations."