SAN FRANCISCO -- In this tech-savvy city teeming with commuters and tourists, the cellphone has become a top target of robbers who use stealth, force and sometimes guns.

Nearly half of all robberies in San Francisco this year are cellphone-related, police say, and most occur on bustling transit lines.

One thief recently snatched a smartphone while sitting behind his unsuspecting victim and darted out the rear of a bus in mere seconds.

Another robber grabbed an iPhone from an oblivious bus rider.

And, in nearby Oakland, City Council candidate Dan Kalb was robbed at gunpoint of his iPhone last week after he attended a neighborhood anti-crime meeting.

These brazen incidents are part of a ubiquitous crime wave striking coast to coast. New York City Police report that more than 40 percent of all robberies now involve cellphones. Thefts of these devices in Los Angeles account for more than a quarter of all the city's robberies.

"This is your modern-day purse snatching," said longtime San Francisco Police Capt. Joe Garrity, who began noticing the trend here about two years ago.

Thefts of cellphones -- particularly smartphones containing everything from photos and music to private emails and bank account statements -- are sending law enforcement agencies and wireless carriers nationwide scrambling for solutions.

When the iPhone 5 went on sale last month, New York City police encouraged buyers to register the serial numbers with the department. That came just months after a 26-year-old chef at the Museum of Modern Art was killed for his iPhone.

In St. Louis, city leaders have proposed an ordinance requiring anyone who resells cellphones to obtain a secondhand dealers license. Resellers would need to record the phone's identity number and collect detailed information including the seller's names, addresses, a copy of their driver's licenses -- even thumbprints.

"It will take a national solution to make this problem go away," St. Louis Mayor Francis Slay said of the phone thefts.

Some experts put annual cellphone losses in the billions of dollars.

Carriers are expected to launch individual databases later this month to permanently disable a cellphone reported stolen. The initiative is similar to a successful decade-old strategy in Australia.

Previously, U.S. carriers have only been able to disable so-called SIM cards, which can be swapped in and out of phones. That's led to a profitable black market for stolen phones.