At least 18 people are killed in the shooting which is being blamed on the Los Tequileros crime gang.
Gunmen have killed the mayor of the city of San Miguel Totolapan, in south-western Mexico, and at least 18 others, officials have confirmed.
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.
In a preliminary report seen by local media, the attorney general of the state of Guerrero, where the city of San Miguel Totolapan lies, reported that 18 people were killed and three were wounded.
Following the attack, the Ministry of National Defence said it was deploying army units to the area to find the gunmen.
Guerrero Governor Evelyn Salgado Pineda tweeted to say she deeply regretted 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 – and was known for threatening mayors of the region – 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.