Mexican leftist Andres Manuel Lopez Obrador, runner-up to President Enrique Pena Nieto in last year's election, suffered a heart attack early on Tuesday and was admitted to hospital, but is improving, doctors said.
Lopez Obrador has been spear-heading protests against Pena Nieto's push to open up the state-controlled oil sector, a central plank of the president's wider economic reform drive. Patricio Ortiz, the cardiologist who attended Lopez Obrador, told a news conference that he was making "satisfactory progress" and was conscious.
He could not say how long Lopez Obrador would remain in hospital. Mexico's peso currency rallied after news of the leftist's hospitalization, and one New York-based economist, who declined to be named, pointed to the possibility that the his health problems could weaken protests against the energy reform.
Giving a televised address on education, Pena Nieto expressed his concern and wished Lopez Obrador a quick recovery. A fiery orator and icon of the Mexican left, Lopez Obrador's closest brush with the presidency came in 2006, when he was narrowly defeated by conservative Felipe Calderon, and spent much of the next six years saying he had been robbed of victory.
Lopez Obrador made the same accusation against Pena Nieto last year and has sought to build a popular front against the energy reform plan, claiming the president wants to sell off Mexico's assets by luring foreign capital to the oil industry.
The reform is due to be debated in Congress in the next few days and is expected to pass before Christmas. Lopez Obrador has sworn he will work to undermine any contracts between the government and private oil companies.
Since losing to Calderon in 2006, when his protests against the outcome brought much of the capital to a standstill for weeks, the silver-haired Lopez Obrador has kept up a hectic schedule, touring Mexico in a state of near-permanent campaign.
Days ago, Lopez Obrador vowed to form a human circle around Congress to protest against the energy reform. Lopez Obrador's son, Andres Manuel Lopez Beltran, told a news conference that the protest called by his father would go ahead in his absence and invited demonstrators to assemble at the Senate on Wednesday morning.
However, recent protests have failed to muster the scale of support he achieved in 2006 when hundreds of thousands took to the streets of Mexico City.