MEXICO CITY (AP) — Mexican President Andrés Manuel López Obrador held a massive rally in Mexico City's main plaza attended by tens of thousands of people Saturday.

Though it was called to commemorate Mexico's 1938 expropriation of the oil industry, many of those attending the rally Saturday agreed that it was the de-facto opening salvo to the 2024 elections that will choose the president's successor.

Perhaps conscious of recent tensions with the United States over US overdose deaths from fentanyl smuggled in from Mexico, López Obrador spent part of his speech praising former US Franklin Delano Roosevelt, who didn't actively oppose the 1938 oil expropriation despite the fact many of the firms were American.

""The best example of the authenticity of his 'Good Neighbour' policy was his respect for our nation's sovereignty," López Obrador said of Roosevelt.

It may be one of the last rallies that will be headed by López Obrador, who is known for his folksy style and charisma. The process to nominate a presidential candidate for his Morena party will begin later this year. After that, the party's candidate is likely to take centre stage.

But most agree that few of the presidential hopefuls can match the popularity of a president whose approval ratings are routinely above 60 per cent. That is especially true for the Morena party, which was largely built around López Obrador.

Alberto Martínez, 59, said he hoped Mexico City Mayor Claudia Sheinbaum would be the party's nominee. "We like her education, her prudence," Martinez said. But he would settle for anyone Morena choose.

Most polls show Sheinbaum as the front-runner in the race, followed by Foreign Relations Secretary Marcelo Ebrard.

"The important thing is for the ideology of López Obrador to continue," Martínez said. "This train is already in motion, somebody just to get aboard and drive it."

Former President Lázaro Cárdenas, one of López Obrador's heroes, delighted Mexicans when he expropriated the largely foreign-owned, privately operated oil industry on March 18, 1938.

One of López Obrador's main policy initiatives has been to save the state-owned oil company that Cárdenas founded from crushing debt and low oil production.

Those attending the rally in the Zocalo wholeheartedly approved of López Obrador, who has struck a nationalist stance, drastically reducing the ability of US anti-drug agents to operate in Mexico.

Blas Ramos, 69, an electrical engineer, held up a sign reading "Get out of Mexico, FBI, CIA, Gringos!"

He said the president was right to oppose US calls to designate Mexican drug cartels as terrorist organisations or to use the US military to crack down on the gangs.

"They are hypocrites," he said of US politicians calling for such measures, "because they don't do anything to reduce drug consumption" in the United States.

The synthetic opioid fentanyl, which kills about 70,000 Americans per year, is mainly manufactured in Mexico with precursor chemicals smuggled in from China.

López Obrador has claimed that Mexico doesn't produce fentanyl — something most experts disagree with — and that the US has a fentanyl problem because American families don't hug their kids enough.

López Obrador thundered against the US proposals. "Mexico is a free and independent country, not a colony or protectorate of the United States," he said, shouting: "Cooperation, yes, submission, no!"

Ramos was confident that the president's movement, which he calls "the fourth transformation of Mexico," would not end when he leaves office in September 2024.

"This is a movement that began a long time ago," he said. "We have spent our whole lives waiting for this movement."

"This movement isn't over in six years," Ramos said, referring to the length of presidential terms in Mexico. "This is a process that will take 30, 40 years."