Armed men opened fire and hurled a grenade into a crowded nightclub early Saturday, killing six people and wounding at least 37 in a western city whose former tranquility has been shattered by escalating battles among drug cartels. The attack in Mexico's second-largest municipality took place just hours after a shootout between soldiers and presumed cartel gunmen left eight people, including an innocent driver, dead in the northeastern city of Monterrey. Monterrey is Mexico's third-largest city. In the Guadalajara attack, assailants in a Jeep Cherokee and a taxi drove up to the Butter Club, located in a bar and restaurant district popular with young people, and sprayed it with bullets. Some of the men then got out of the taxi and threw a grenade into the nightclub entrance, said a police official. The gunmen fled after the pre-dawn attack, he said. Three were killed at the scene and three more died later in hospitals, said Medical Services Director Yannick Nordin. A Venezuelan and a Colombian were among the dead. While there have been isolated grenade attacks around the city, Saturday's was the first to be thrown into a crowd and cause so many injuries. The U.S. Consulate in Guadalajara recently warned U.S. citizens not to drive at night in parts of the city after suspected drug-gang members burned vehicles and blocked streets. Such alerts have become common for highways in some areas of northern and western Mexico, but not for Guadalajara, which is known more for its mariachi music and tequila than as a focal point of a drug war that has claimed nearly 35,000 lives since 2006.