The little-known guerrillas stunned the nation with a series of co-ordinated attacks on official buildings in six states during the night of Wednesday to Thursday, leaving 15 people dead and scores wounded.
In communiques they left behind, they called for the overthrow of the government of the Institutional Revolutionary Party (PRI), which has ruled Mexico since 1929, and said they would never negotiate with a government of that party.
Although President Ernesto Zedillo, who is due to give his annual Informe (State of the Nation address) to Congress tomorrow, continued to play down the attacks as "isolated incidents", the feeling of shock throughout Mexico was tangible. The stock market took its sixth-worst fall of the year after the attacks and the peso slid seven centavos against the dollar.
The Finance Minister, Guillermo Ortiz, issued a statement aimed at calming the markets, saying Mexico would continue to pay its foreign debts despite calls by the guerrillas to default.
The country's business leaders called for tough action to root out the rebels, saying the attacks could badly affect tourism and add to the unemployment problem. "The government should not fight these people with holy water," the statement said.
The attacks, far more widespread and organised than those by Mayan Indian rebels of the Zapatista National Liberation Army (EZLN) in the state of Chiapas in 1994, indicated for the first time that leftwing guerrillas from various groups had linked up through much of poverty-stricken central and southern Mexico.
The Zapatistas caught security forces off guard on New Year's night 1994, taking five Chiapas towns for a day or two before fleeing back to the Lacandon jungle and eventually entering into peace talks. The EPR struck in six states, hundreds of miles apart, at more or less the same time, showing organisation, training and weaponry the Zapatistas never had. No one seriously suggested Mexico was on the brink of revolution but the attacks were serious enough for some to at least recall the bloody revolution of 1910-1919.
"We are not in a state of revolution," the country's business leaders said in their statement. "There is nothing to be gained by saying that we are not in a situation like 1910," responded a cryptic front-page editorial in yesterday's evening edition of the big daily Excelsior.
With a growing sentiment that Mr Zedillos's free-market economic policies are simply widening the gap between Mexico's rich and poor, the emergence of a widely operating leftist guerrilla group could be the biggest challenge the once all-powerful PRI has ever faced.
Worrying many was the fact that the EPR's communiques expressed support for a movement called El Barzon (The Yoke), a group of hundreds of thousands of middle-class Mexicans who have been protesting for more than a year over the government's economic policies.
The government conceded for the first time that the guerrillas appeared to have emerged as an armed force with roots in an urban guerrilla uprising which followed repression of student riots in the years after 1968. It said the EPR seemed to have emerged from two older groups: the Clandestine Workers' Revolutionary Party (Procup), students and intellectuals-cum- urban guerrillas who launched minor attacks since the Seventies in Mexico City and elsewhere, and the Party of the Poor (PDLP), a peasants'-rights group crushed by the army in the state of Guerrero in the Seventies.
"These people are a hangover from the Seventies," the Interior Ministry spokesman, Arturo Nunez, said. "They wanted to give the appearance of a nationwide presence before President Zedillo's speech. We will certainly not enter into dialogue with terrorists and criminals."
The 80 masked rebels who attacked four government installations in Huatulco, a favourite destination for American, Canadian and European tourists, vanished into the lush hills above the resort after firing hundreds of rounds at local and state police stations, a small naval installation and the headquarters of the local Prosecutor-General's office.
The history of a violent region
Guerrero has seen some of the blackest episodes of repression in Mexico's history and spawned several of the most combative armed movements of the Sixties and Seventies. Here the legendary Genero Vazquez and Lucio Cabanas led brutally suppressed rural uprisings in the first half of the Seventies. Guerrero, Oaxaca and Chiapas are all victims of the endemic political neglect that has been the breeding ground for social conflict. Another armed group of the Seventies, the Forces of National Liberation, flourished in Chiapas and developed in 1994 into the Zapatista Army of National Liberation.