The ruling National Regeneration Movement (MORENA) declared Sheinbaum the winning candidate a few hours after her closest competitor, ex-foreign minister Marcelo Ebrard, said the primary process had been marred by irregularities.

President Andres Manuel Lopez Obrador speaking at a regular press conference extended a hand to Ebrard, calling him a "very good person," a good leader and his friend, as he urged him to remain united with the leftist MORENA.

Lopez Obrador rejected Ebrard's demand that the primary be redone, and said he supported Sheinbaum.

"I celebrate what was done yesterday. It's something historic, unprecedented, and I don't see any problem," Lopez Obrador said about the process where Sheinbaum beat out five other contenders.

Ebrard on Wednesday said he would decide his next move on Monday and left open the possibility of joining forces with the center-left Citizens' Movement (MC), the only significant opposition party yet to back a candidate for the June 2 presidential election.

Lopez Obrador suggested Ebrard may choose to run for the presidency as an independent candidate, adding that he is free to do what he considers best.

Latest opinion polls suggest that MORENA is heavily favored to win the presidency again, bolstered by Lopez Obrador's personal popularity.

"This type of process has never been done," Lopez Obrador said, referring to the polls of some 12,500 people that resulted in Sheinbaum becoming the MORENA candidate.

"(Sheinbaum) is an honest woman with convictions and principles," the president added.

Victory would make her Mexico's first woman president. The main opposition alliance has already chosen a female candidate - Xochitl Galvez, a charismatic and unconventional senator of Indigenous origin who overcame an impoverished background to become a successful entrepreneur.

Under Mexican law, presidents can only serve a single six-year term.