"The first reports are of at least 20" cadavers, said Lino Gonzalez, spokesperson for the prosecutor's office in the western state of Jalisco. Guadalajara, population 4.4 million, is the state capital.

The victims had been tied and gagged, senior Jalisco state official Luis Carlos Najera told Televisa network. He spoke of "more than 20" victims.

One of the vehicles was a compact car, another was a cargo truck and the third was an SUV, Najera said.

Inside one of the vehicles, police found a message from an alleged criminal gang. Najera, however, did not release the content because of the ongoing investigation.

The Guadalajara bodies were discovered one day after two vehicles containing 16 charred bodies were found in Culiacan, capital of the violence-wracked northwestern state of Sinaloa, in what officials called a macabre message between rival drug gangs.

The La Familia drug cartel, based in neighboring Michoacan state, is active in Jalisco -- but so is the rival Sinaloa gang headed by one of Mexico's best-known drug lords, Joaquin "El Chapo" (Shorty) Guzman.

The vehicles were found at a busy intersection in downtown Guadalajara, close to the city's convention center.

This weekend, the International Book Festival, the most important book in the Spanish-language world, takes place at the convention center. Scores of authors, including Nobel laureates in literature Mario Vargas Llosa and Herta Muller, are expected at the event.