The bodies of 15 men, all but one of them headless, were found on a street outside a shopping centre in the Mexican tourist holiday resort of Acapulco.
The decapitated victims, all of whom appeared to be in their 20s, were discovered in an area not usually frequented by tourists.
Local police said 27 people were slain in the Pacific resort in one day over the weekend.
Handwritten signs left with the bodies were signed by "El Chapo's People" -- a reference to the Sinaloa cartel, headed by drug lord Joaquin 'El Chapo' Guzman -- said Fernando Monreal, director of investigative police for Guerrero state, where Acapulco is located.
The narco-messages indicated the Sinaloa cartel killed them for trying to intrude on the gang's turf and extort residents. Authorities said there were at least three minors among the dead.
Mexico's drug cartels have increasingly taken to beheading their victims in a grisly show of force, but Saturday's discovery was the largest single group of decapitation victims found in recent years.
In 2008, a group of 12 decapitated bodies were piled outside the Yucatan state capital of Merida.
The same year, nine headless men were discovered in the Guerrero state capital of Chilpancingo.
Acapulco has been the site of fierce battles between drug gangs, and the weekend was a bloody one, with 27 people killed there from Friday evening to early Saturday, Mr Monreal said.
The dead included two police officers cut down on a main bay-side avenue in front of tourists and locals; six people who were shot dead and stuffed in a taxi, their hands and feet bound; and four others elsewhere in the city.
Two police officers were wounded when armed men attacked a police post in the city's Emiliano Zapata district.
"We are co-ordinating with federal forces and local police to reinforce security in Acapulco and investigating to try to establish the motive and perpetrators of these incidents," Mr Monreal said.
The wave of violence in one of Mexico's biggest resorts was condemned by the federal government.
"Reprehensible acts of violence such as these underscore the need to fight with determination against organised crime," a statement from Mexico's interior ministry said.
At least 30,196 people have died in drug-related violence since Mexican President Felipe Calderon launched an offensive against cartels in late 2006.