Police are warning against using iPhones in public in Miami Beach.

It's a pretty dire directive for a city full of Instagramming visitors, but a rash of armed robberies have officials concerned Miami Beach is another frontline in the global black market for stolen iPhones.

Last week, two Venezuelan tourists were walking to their building on Collins Avenue when three armed men forced them against a wall and ran off with an iPod. Blocks away, two women were robbed of their cash and iPhones at gunpoint after exiting a cab.

The increasingly popular crime is known as Apple picking because crooks target unsuspecting iPhone users holding their phones in plain sight.

"The subjects will look when people are talking on their iPhone or not paying attention, in this case with a firearm, and will take the phone right out of their hand and take off," Miami Beach Police spokesperson Vivian Hernandez told NBC 6.

The market for stolen Apple phones, which are typically shipped and sold in Latin America, Asia, and Eastern Europe, is worth some $30 billion a year, according to the mobile security firm Lookout.

WSVN reports eight arrests were made in Miami Beach over the weekend. The same suspects, all between the ages of 18 and 21, are currently being investigated for several iPhone theft incidents in Coral Gables and Miami.

Peter Trubetskoy was robbed while waiting for a friend outside the South Beach nightclub Mokai.

"Two guys approached me, and they said 'Hello' to me, 'Hi,'" he told WSVN. "I was polite to them, and they walked away from me. They came back with another guy who pointed a gun to my head and asked for my cellphone."

Apple picking has already juiced up crime statistics in New York City, where it spiked an increase in last year's overall crime rate. In Washington, D.C., officials reported a 54 percent jump in cell phone robberies from 2007 to 2011.

The NYPD is even teaming with Apple to improve features that can locate stolen iPhones and iPads anywhere in the world.

In Miami Beach, police say those with iPhones should be particularly vigilant "late at night or in the early morning hours" -- and encourage users to enable location features such as Find My iPhone.

They also say users should enable the iPhone's passlock, and never leave a phone out on the table when dining in a restaurant.