Mexico City: Authorities report that at least 17 others, including the mayor of a small town in western Mexico, have been killed by gunmen.
According to police, armed men stormed the San Miguel Totolapan town hall at 14:00 (19:00 GMT) on Wednesday. Photos posted online show how many bullets it is riddled with.
The left-leaning PRD party called for justice for the "cowardly" murder of Mayor Conrado Mendoza Almeida.
Los Tequileros, a criminal organization affiliated with a powerful drug cartel, has been charged with carrying out the attack.
The massacre also claimed the lives of police officers and council staff, with graphic images of blood-soaked bodies lying on the ground on social media.
Mr. Mendoza Almeida's father, Juan Mendoza Acosta, also died in his home before the attack broke out at the town hall.
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Large vehicles reportedly blocked a highway in Guerrero state, where San Miguel Totolapan is located, to prevent security forces from entering the city.
According to BBC Mexico correspondent Will Grant, the attack was shocking even by the violent standards of the state of Guerrero.
The drug cartels' control includes San Miguel Totolapan in Tierra Caliente, a violent region in western Mexico.
Our correspondent continued: "Various groups engage in struggle for control of lucrative drug routes to the north along the Pacific Corridor.
Guerrero's attorney general said that in addition to the 18 deaths, three other people were also injured in preliminary reports provided to local media.
After the attack, the Defense Ministry announced that Army and Navy units were being sent to the area to trace the shooters.
Guerrero's governor, Evelyn Salgado Pineda, tweeted her deep regret over the deaths.
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The Los Tequileros drug gang, which was engaged in a clash with another nearby drug gang, announced its return to the area in a video posted on social media just before the attack.
The criminal organization that ravaged Guerrero between 2015 and 2017 was known to bully local mayors prior to the assassination of Rebel Jacobo de Almonte.
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De Almonte's gang adopted the nickname "El Tequilaro" (Tequila Drinker) from him.