As shoppers descended on malls and crowded D.C. streets for the "Black Friday" bonanza that traditionally kicks off the holiday shopping season, police departments ramped up their tactics to combat the light-fingered thieves who appear this time of year as reliably as twinkle lights and holly.

The move to combat pickpockets -- long considered merely annoyance criminals -- has gained new urgency in recent years as thieves have changed their focus from cold hard cash to identity theft, police said.

"Pickpocketing has changed so much," said Metro Transit Police Detective Cedric Mitchell. "In the old days, they'd pick your wallet, take the cash and dump the wallet in the trash. Now they realize they're throwing more money away than they're taking because of the identity theft portion of the crime."

He said thieves now realize that the wallet can be a gold mine of information for those looking to do serious damage.

Mitchell said he is in the midst of a case in which a young woman's wallet was stolen in a Metro station, and the thieves have racked up $20,000 in charges on credit cards they obtained by using the personal information they found.

To deter such incidents, Fairfax County police yesterday began deploying squads with two to 10 "Christmas anti-theft" teams of undercover officers at the Springfield, Tysons Corner and Fair Oaks malls, Officer Bud Walker said. In Prince George's County, uniformed and plainclothes police were to begin patrolling 44 shopping centers today and passing out pamphlets to advise customers on how to protect themselves.

"We're going to be very visible this holiday season," said Cpl. Kim Brown, a Prince George's police spokeswoman.

The District has assigned a special holiday detail of officers to shopping districts along Connecticut and Wisconsin avenues NW, including Mazza Gallerie and the shops in and around Georgetown. Lt. Felicia Lucas said the purpose is to have "high-visibility patrols" to deter criminals.

Lucas said the department is also asking residents to be alert.

"They should be cautious in dimly lit areas and not talk on cell phones or listen to iPods while walking," she said. "We also urge them not to leave items such as electronics or laptops in plain view while they're shopping. You never know who is watching."

Robert Siciliano, an identity theft expert, said the money and credit cards in a wallet are nearly incidental compared with the far more valuable information that can be extracted, particularly if a Social Security number is on an insurance card or driver's license.

"If you show ID for any reason, and your driver's license number is your Social Security number, then you're giving away the keys to the kingdom," Siciliano said.

Another no-no: writing down your ATM pass code on a piece of paper in your wallet or, worse yet, on the card itself. (Yes, he said, people do it.)

Derek Baliles, a spokesman for the Montgomery County police, said pickpockets tend to target the elderly. One classic move is the bump-and-grab, in which the thieves jostle a mark and come away with the goods. Others work in teams -- one to strike up a pleasant conversation with a victim while the other steals.

Metro Transit Police recently issued a warning to riders that pickpocket crime tends to increase this time of the year because thieves are looking for ready cash from riders distracted by the holiday hubbub. The system had 109 pickpocket incidents last year; 71 have occurred this year, said spokeswoman Lisa Farbstein.

Most recently, a rider was in the middle of the Gallery Place Metro station when she felt an odd twinge uncomfortably near her purse. She whirled around and saw another woman standing there -- holding her wallet. A screaming match ensued. A Metro officer rushed over and ultimately arrested a 37-year-old District woman and charged her with one count of felony robbery.

Police and security experts advise carrying only the essentials in your wallet -- a few credit cards and cash. Women should keep their purses zipped. Men should avoid putting their wallets in their back pockets, instead using a front or coat pocket or dispensing with a wallet in favor of a money clip.

Shoppers at malls around the region yesterday showed varying degrees of care with their personal belongings -- some stringent, some lackadaisical.

Shopping in the District, Glenn Mas, 37, a graduate student at Catholic University, said his security system was unbeatable.

He wears his backpack in front, with his credit cards and identification nestled in a "secret compartment,'' and keeps his watch covered by his shirt sleeve.

"I keep only what I need -- my Metro fare -- in my front pocket,'' he said while heading to catch a train at Metro Center.

Shopping in Virginia yesterday, Charles Chavez of Manassas had his wallet in his back pocket, like most men. "I've never thought about it," he said. He said when he travels out of the country, he usually moves his wallet to his front pocket, "but when I'm here in the States, you never think about it. You have much more trust."

Staff writers Eric M. Weiss, Robert Pierre and Tom Jackman contributed to this report.

Shoppers yesterday at Tysons Corner Center. The Fairfax County Police Department has begun deploying undercover anti-theft officers at malls.