Mexican President Andres Manuel Lopez Obrador claimed victory in a divisive referendum he initiated to decide on whether he should step down or complete his term, amid a low turnout.

With an approval rating of nearly 60%, Lopez Obrador's presidency had never appeared seriously at risk, and the left-wing populist was himself one of the vote's biggest cheerleaders.

The 68-year-old president, elected in 2018 for a six-year term, won slightly more than 90% of votes in favor of his staying in office until 2024, according to an official initial count.

But the turnout rate was estimated to be no more than around 18%, well below the 40% level needed for the poll to be legally binding.

That means that even if Lopez Obrador had lost, he would not have been obliged to step down.

Lopez Obrador ruled out using the result to pursue a constitutional reform allowing him to seek another term, as some critics have accused him of planning to do.

"I'm going to continue serving until the last day of my term. I'm not going to go past that because I'm a democrat and I'm not in favor of re-election," he said in a video message.

The 68-year-old president, who was elected in 2018, cast his vote in front of the press with his wife at a polling station near the presidential palace.

As promised, he intentionally spoiled his ballot with the words "Long live Zapata!" in homage to a Mexican revolutionary hero.

"Let no one forget that the people are in charge," declared Lopez Obrador, who had vowed not to vote for himself to remain impartial but said it was important to take part.

Supporters of the referendum – the first of its kind in Mexico – said it was a way of increasing democratic accountability, giving voters the opportunity to remove the president due to loss of confidence.

"Now we have the chance to change what's not right. There have been presidents who, after being elected by the people, ended up serving other interests," Benigno Gasca, a 57-year-old mathematician and musician, told Agence France-Presse (AFP).

But critics see it as expensive propaganda and an unnecessary distraction from the many challenges facing the country, including drug-related violence, poverty and the rising cost of living.

"It's a useless exercise – money thrown away," said Laura Gonzalez, a 62-year-old retired teacher.

President Lopez Obrador, a pugnacious leftist who has governed since December 2018, is the architect of the first so-called recall referendum in modern Mexico, and critics and supporters alike expected him to win easily.

The vote has fueled speculation it could open the door to extending presidential term limits in a country where the head of state is allowed to serve only a single six-year period.

Lopez Obrador denies he wants to extend his term, but he has used the referendum to fire up supporters and pan the opposition, many of whose leaders have encouraged Mexicans to ignore the vote as a propaganda exercise for the president.

Opinion polls suggest that turnout is likely to fall short of the 40% threshold required to make the ballot binding, although Lopez Obrador has said he will respect the outcome regardless.