Gunmen have killed the mayor of the city of San Miguel Totolapan, in south-western Mexico, and at least nine others, local media has reported.
Police say gunmen attacked the city hall at 14:00 (19:00GMT) on Wednesday, with photos posted online showing the building riddled with bullet holes.
Mayor Conrado Mendoza Almeda's party, the PRD, has condemned his "cowardly" assassination and demanded justice.
The attack has been blamed on the Los Tequileros criminal gang.
Police officers and council workers were also killed in the massacre, with graphic images of bloodied bodies lying on the ground circulating on social media.
There are some reports that Mr Mendoza Almeda's father, former mayor Juan Mendoza Acosta, was also killed - in his home - before the attack moved to the city hall.
The total number of victims varies between reports, and has not been confirmed by authorities.
Following the attack, the Ministry of National Defence said it was deploying army units to the area to find the gunmen.
Evelyn Salgado Pineda, governor of the state of Guerrero - where the city of San Miguel Totolapan lies - tweeted to say she deeply regrets the deaths.
Shortly before the attack, alleged members of Los Tequileros released a video on social networks announcing its return to the region.
The criminal group devastated Guerrero between 2015 and 2017, until its leader, Raybel Jacobo de Almonte was assassinated.
De Almonte was known as El Tequilero - the Tequila Drinker - and his gang took their name from him.
