Mexican President Andres Manuel Lopez Obrador has staged a massive rally in Mexico City’s main plaza attended by tens of thousands of people.

Though it was called to commemorate Mexico’s 1938 expropriation of the oil industry, many of those attending the event on Saturday agreed 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 overdose deaths in America from fentanyl smuggled in from Mexico, Mr Lopez Obrador spent part of his speech praising former president Franklin Roosevelt, who did not actively oppose the 1938 oil expropriation despite the fact many of the firms were American.

Mr Lopez Obrador said of Roosevelt: “The best example of the authenticity of his ‘Good Neighbour’ policy was his respect for our nation’s sovereignty.”

It may be one of the last rallies that will be headed by Mr Lopez 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, and at that point the 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%. That is especially true for the Morena party, which was largely built around Mr Lopez Obrador.

Rally attendee Alberto Martinez, 59, said he hopes Mexico City mayor Claudia Sheinbaum will be the party’s nominee. “We like her education, her prudence,” he said – though he vowed to back anyone Morena chooses.

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

“The important thing is for the ideology of Lopez Obrador to continue,” Mr Martinez said. “This train is already in motion, somebody just needs to get aboard and drive it.”

Former President Lazaro Cardenas, one of Mr Lopez Obrador’s heroes, delighted Mexicans when he expropriated the largely foreign-owned, privately operated oil industry on March 18, 1938.

One of Mr Lopez Obrador’s main policy initiatives has been to save the state-owned oil company that Mr Cardenas founded from crushing debt and low oil production.

Those attending the rally in the Zocalo wholeheartedly approved of Mr Lopez 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 is right to oppose US calls to designate Mexican drug cartels as terrorist organisations or to use the American 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 their own country.

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

Mr Lopez Obrador has claimed that Mexico does not produce fentanyl – something most experts disagree with – and that the US has a fentanyl problem because American families do not hug their children enough.

Mr Lopez Obrador thundered against the US proposals at the rally: “Mexico is a free and independent country, not a colony or protectorate of the United States.

“Co-operation, yes, submission, no.”