Be warned, this video is extremely graphic -- Alfredo Cabrera is seen smiling and greeting supporters in the middle of his final campaign rally stop in Coyuca de Benitez, Guerrero, Wednesday ... just moments before he's brutally shot, which was being filmed.

The perpetrator isn't fully visible in the clip ... but you can see a gun in their hand, boldly pointed at the back of Cabrera's head just as he's about to address 300 supporters -- and then they fire.

The video gets fuzzy, but you can hear the sound of several more gunshots in succession, along with the panicked screams of terrified supporters.

Another distressing video captured later shows Cabrera's lifeless body lying on the ground at the abandoned campaign rally event ... and you also see the body of the purported gunman.

The current Gov. of Guerrero, Evelyn Salgado, condemned the crime as cowardly in a statement ... saying she's urged the Attorney General's Office of the State of Guerrero to conduct a thorough investigation and ensure the full weight of the law is applied to the gunman and all those responsible for the crime.

However, according to the prosecutor's office, the alleged attacker was killed at the scene.

Cabrera's slaying adds to the grim tally of murdered candidates, mostly vying for mayor or town councils, during Mexico's 2024 election cycle, now reaching 36 -- making it the bloodiest election in the country's history.