Mr Camacho assailed the PRI, which has ruled Mexico since 1929, and implied he would form a new centrist coalition to run for president in 2000 - if the incumbent President Ernesto Zedillo lasts that long.
"I am already out of the PRI," Mr Camacho said in a brief statement. "I am in favour of real political change, a new political coalition, to lead us to an advanced democracy."
Political commentators in Mexico City said the defection of Mr Camacho, who only two years ago was widely tipped to be Mexico's next president, was a major blow to a party already rotting from within and showing signs of crumbling before it reaches 70 years in power.
Mr Camacho is the most prominent PRI member to desert since Cuauhtemoc Cardenas left in 1988 to launch a centre-left coalition, which became the Democratic Revolution Party. That defection stunned the nation. Mr Cardenas ran for president and many, if not most, Mexicans believe he defeated the PRI candidate, Carlos Salinas de Gortari, only to be robbed of victory by fraud.
Mr Cardenas's popularity, strong among poor Indians and peasants who revered his part-Indian father, President Lazaro Cardenas, has since slipped, as workers and middle-class voters feared his populist policies.
Mr Camacho may well believe he can garner much of his support on the left, attract the moderate wing of the rising conservative National Action Party and even create a new party from other PRI dissidents and supporters.
Considered one of Mexico's shrewdest politicians - far more so than Mr Zedillo, a stand-in presidential candidate after the assassination of Luis Donaldo Colosio in March 1994 - Mr Camacho's timing could be a destructive blow to the PRI.
Some Mexican analysts wondered whether Washington, whose tentacles are never far from Mexican politics, was pushing Mr Camacho and a possible new centrist party as a catalyst for peaceful transition, at a time when the old ruling party shows signs of dying on its feet.
Mr Camacho's main problems may be his ego and ambition. After Mr Salinas, his old friend and mentor, passed over him and chose Mr Colosio as the PRI presidential candidate in 1993, Mr Camacho broke the party's traditional rule of silence and criticised the decision.
Still, Mr Salinas named him a peace negotiator with Zapatista guerrillas after the January 1994 uprising in the south-eastern state of Chiapas. The timing of the uprising and the assassination of Mr Colosio two months later led to a spate of conspiracy theories, some involving Mr Camacho, the now-disgraced Mr Salinas and/or long-time PRI hardliners.
During the initial Chiapas peace talks, Mr Camacho was receiving far more publicity than Mr Zedillo, then the presidential candidate. Possibly fearing some kind of coup against his candidacy, Mr Zedillo criticised Mr Camacho for his cosiness with the Zapatista leader, Subcomandante Marcos. An angry Mr Camacho quit as peace negotiator and laid low until this weekend.Reuse content