Andres Manuel Lopez Obrador, Mexico's leading figure on the left, underwent surgery to implant a stent after suffering a heart attack early Tuesday, doctors at a capital hospital said.
The two-time presidential candidate walked into the hospital at 2:30 a.m. complaining of chest pain and was found to have suffered an "acute myocardial infarction," Dr. Patricio Ortiz told a press conference.
Lopez Obrador, 60, was awake and alert following the procedure, the physician said.
Neither Ortiz nor the hospital's medical director, Dr. Octavio Gonzalez, offered an estimate of how long the former Mexico City mayor will have to remain hospitalized.
The doctors were joined at the news conference by the patient's son, Andres Manuel Lopez Beltran, who said the protest his father had called for Wednesday outside the Mexican Senate would take place as planned.
Lopez Obrador, who is in the process of converting his National Regeneration Movement, or Morena, into a formal political party, announced last Sunday that he would lead a demonstration outside the Senate as senators were due to take up a proposed overhaul of Mexico's oil and gas industry.
Both Morena and the center-left PRD party oppose the plan to open the sector to private investment, which they see as a step toward privatizing state-owned Petroleos Mexicanos.
Morena supporters should gather at the Senate at mid-morning on Wednesday, Lopez Beltran said.
Lopez Obrador finished second in the 2006 and 2012 presidential elections.
Conservative Felipe Calderon's margin of victory over Lopez Obrador in 2006 was 0.56 percent, the smallest in Mexican history, and the leftist refused to recognize the result.
The man known as AMLO was defeated last year by Enrique Peña Nieto, of the Institutional Revolutionary Party, or PRI, who issued a statement Tuesday expressing his "greatest solidarity" with the ailing Lopez Obrador.
In power from 1929-2000, the PRI began to splinter in the late 1980s with the departure of Cuauhtemoc Cardenas - son of President Lazaro Cardenas, who nationalized the oil industry in 1940 - to form the PRD.
Among those who followed Cardenas out of the PRI was Lopez Obrador.
AMLO was the PRD standard-bearer in 2006 and 2012, but left the party about a year ago, unhappy with the leadership's willingness to cooperate with the PRI. EFE