The growing violence in Monterrey, long one of Mexico's most modern and safe cities, is a sign that the country's war against drug gangs is spreading ever further from poorer battlegrounds along the border and into the country's wealthiest enclaves.

Residents opened their newspapers Wednesday morning to find the ads taken out by Mexican business leaders, begging the government to send more military into the city. "Enough already," said the notice that ran in national and local papers, criticizing what it said was a slow response of police against "criminal bands that in every act look to establish a new boundary of terror."

Later that day, the body of Edelmiro Cavazos, mayor of the Monterrey suburb of Santiago, was found beside a highway. Mr. Cavazos had been abducted Sunday night, the latest in a string of attacks against politicians in Mexico's north.

His killing is another incident in a terrifying spell for Monterrey residents that began Saturday when armed gangs set up more than a dozen roadblocks on key boulevards of the city, paralyzing traffic for hours. The next day, a grenade was lobbed at the offices of an important television broadcaster. On Tuesday night, grenades were also hurled at several small businesses on the city's outskirts.

"The security environment in Monterrey has turned, in just a few months, from seeming benevolence to extreme violence," U.S. Ambassador to Mexico Carlos Pascual said at a recent conference on drug trafficking in El Paso, Texas.

Monterrey is only the latest sign of mounting problems in Mexico's war on organized crime. The official toll since President Calderón took power in December 2006 is more than 28,000, according to government figures through July. Mr. Calderón recently acknowledged the government's inability to check the violence with brute force alone and has invited lawmakers to debate measures such as legalizing drugs.

The brutality of the conflict is escalating. Alleged gang hit men broke into the home of a Chihuahua state policeman this week and strangled to death his 4-year-old brother, authorities said. Across the country, mutilated and decapitated bodies turn up virtually every day, sometimes hanging from bridges.

It wasn't always this way in Monterrey. Mexicans know the city of 2 million as "The Sultan of the North," a nickname stemming from its wealth, generated by corporations such as Mexico's beverage titan Femsa S.A.B. de C.V. and U.S. businesses such as Corp. and General Electric Co., which have large offices in Monterrey. International architects built the city's skyline, which is framed by dramatic mountains.

Security forces find the body of a mayor days after he was abducted by hitmen in the latest attack on a public official by increasingly bold drug gangs. Video courtesy of Reuters.

The wealth created a sense that Monterrey was impervious to the drug war. "People just didn't think it was going to happen here," said Carlos Jáuregui, who until March was the top security officer in Nuevo León state where Monterrey is. "Now most of our police corps have been infiltrated by organized crime."

In April, hooded men raided a Holiday Inn in Monterrey taking several hostages, who remain missing. The month before, two doctorate students were killed as bystanders in a shootout at Mexico's most prestigious university, the Monterrey Institute of Technology and Higher Education, known as Monterrey Tech.

But authorities say Monterrey's high-end neighborhoods have drawn some of country's richest drug lords, particularly from Los Zetas, one of the country's most violent crime groups. In the past, these hideouts were considered "off-limits" to rival gangs. But now it appears the violence has become so out of hand that the drug bosses themselves can no longer control it.

Meanwhile, Monterrey's other well-heeled residents have become targets for violent car theft and kidnappings for ransom—alternative sources of income for crime groups. Fernando García, 62, said he was driving in the city's wealthy suburb of San Pedro Garza García one afternoon in June when he and his son were approached by armed men who ordered them out of their vehicle. One of the men sped off with their car, while two others held Mr. García and his son in a second vehicle for the next seven hours, forcing them to withdraw cash from various automated teller machines, until the cards were blocked after $1,000 in withdrawals. "There were people driving by when it happened, and no one helped us," Mr. García said. Police have made no arrests for the crime.

Another frightening development is the emergence of so-called narcobloqueos, or drug blockades, the tactic used last weekend by drug gangs to snarl traffic in the city. The first occurred at the beginning of this year, and now the city has become known for them.

City leaders recently assembled an emergency towing team to clear the streets after blockades.

As Monterrey's roads have become danger zones, so has its esteemed university, the Monterrey Tech. In March, Javier Arredondo and Jorge Antonio Mercado were killed on the campus after a shootout with soldiers. Initially, the military said the two were hit men. It later surfaced they were unarmed students. Last week, Mexico's National Human Rights Commission said the military had planted weapons on the students. Mexico's military denies having done so.

Ernesto Canales, an attorney from Monterrey, said such incidents undermine public trust. "Now we feel like we're caught between the bad guys and the military," he said.

Javier Treviño, Nuevo León's lieutenant governor since October, said the military presence is necessary because drug cartels have infiltrated local police. Escalating violence in the city shouldn't be interpreted as a sign that drug traffickers are winning, he said, but is rather often the painful result of the state's capture of big cartel leaders this year. "As we dismantle these organizations, there are some consequences. Every time you have new bosses of the different cells, you have new violence associated with that," he said in an interview.

Richard Hildreth, a director at New York-based security consultant Altegrity Risk International, said companies are increasingly seeking advice on how to fend for themselves in the city. "U.S. companies see Monterrey as high-risk now," Mr. Hildreth said. Polaris Industries, a large maker of snowmobiles and all-terrain vehicles in Medina, Minn., that plans to open a manufacturing facility in Monterrey is going ahead with the plant, a spokeswoman says, but recent violence has become a "serious concern."

The consequences of violence drag on into everyday life. Locals say they host parties in the afternoon now, so guests can avoid driving at night. Many Monterrey residents say they won't travel to South Padre Island, a popular resort on the coast of Texas, for fear of passing through Reynosa, a city where hundreds of murders have occurred this year. This summer, Monterrey's high-end Palacio de Hierro shopping mall was held up by gunmen for the third time this year.

"We're defenseless here. Who do we call for help?" said a local hotel owner who used to own the Holiday Inn stormed by masked gunmen in April, asking that his name not be used.