Terrifying footage captured the moment fierce winds ripped through the structure and brought it down as crowds fled.
Rescue operations are still underway to save victims crushed beneath the stage in the northeastern city of San Pedro Garza García.
The governor of Mexico’s Nuevo Leon state Samuel Garcia said one child is among the dead.
He added that some of the injured are now stable but others are undergoing surgery.
Video of the accident showed the structure suddenly falling forward into the crowd, sending the politicians on stage running for cover and panicked attendees scattering.
Governor Garcia warned residents to stay indoors amid strong thunderstorms in the area.
Winds of up to 43mph had been forecast on Wednesday.
Jorge Alvarez Maynez, the presidential candidate for the centrist Citizens’ Movement party, said: “I’ve never experienced something so sudden.”
He was injured in the incident along with members of his election team and taken to hospital briefly.
The politician later said that he would suspend all campaign activities and that he would remain in the state to monitor the situation and the victims.
“We have to have solidarity, there is nothing that can repair an accident, a damage of this nature, and [people] will not be alone in this tragedy,” Maynez added.
Mexican President Andrés Manuel López Obrador said he was sending “a hug to family members, friends of the victims and political supporters”.
Mexico is heading towards its largest election in history on June 2 after months of campaigning blighted with political violence and assassinations.
In total, more than 20,000 positions are being contested.
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