When the threatening phone calls demanding $20,000 in protection money began in December, Dr. Roman Gomez Gaviria shrugged them off, believing his clinic on the outskirts of Mexico City couldn't possibly be of interest to criminal gangs. A few months later, his sense of security was shattered when three armed men barged into his office screaming "Dr. Ramon, you bastard, where are you?"

"They tried to tackle me, to take me out of the clinic, when I saw that each one had a pistol tucked into his belt," said Gaviria, recounting the ordeal. "They thought that, because I'm a doctor, I wasn't going to resist."

Such shakedown rackets have long targeted businesses in the most violent corners of Mexico. Now the practice is spreading. One anti-crime group estimates that kidnapping across the country has jumped by one-third so far this year compared to 2012. And as the extortion industry expands, it has drawn both experienced criminals and imitators.

Experts say the increase is a byproduct of Mexico's crackdown on the nation's drug gangs. As authorities nab cartel bosses and break up chains of command, hundreds of lower-level gunmen and traffickers are desperate for income and looking for income in new places.

Targets include everything from multinational businesses to corner pharmacies and unsuspecting holidaymakers. The gangs are less organized, but more ubiquitous than the drug cartels, affecting broad swaths of the country.

"It affects all economic activity. It discourages investment," said security expert Jorge Chabat.

In the first eight months of 2013, there were 5,335 reported extortion attempts nationwide, equal to the number for all of the previous year. If the current pace continues, the total could surpass 8,000 this year, almost twice as many as in 2007.

The tourism industry, Mexico's third-largest source of foreign revenue, has been one of the hardest hit. Largely untouched when the U.S.-backed drug war began in late 2006, the state of Oaxaca had quietly become the turf of the Zetas cartel. In recent months, guests of at least a dozen hotels in scenic, colonial Oaxaca city have started receiving calls from strangers saying they would be kidnapped if they didn't pay between $380 and $1,500, hotel industry and security officials said.

"The way they operate is to call the hotel, ask to speak to a particular room and then start threatening" the guest, said Joaquin Carrillo Ruiz, an assistant state prosecutor in Oaxaca. Many of the tourists, all from Mexico, reported the crime instead of paying up, but that hasn't calmed worries in Oaxaca, where tourism is a vital source of outside income.

"We have to stop this in its tracks," said Juan Carlos Rivera, the head of the Oaxaca Hotel Association. "If we don't, it could escalate."

As if to prove his point, a group of Spanish musicians were hit by a telephone extortion scheme in Mexico City this month, though none was kidnapped or harmed.

But even authorities acknowledge that the vast majority of extortions go unreported — as many as 92 percent according to a survey of crime victims by the National Statistics Institute. The same survey from April indicated that extortion is now the second most common crime after street robberies, with 7.6 percent of those surveyed in 2012 saying they were extortion victims, up about two percentage points from the year before.