Calling themselves the People's Revolutionary Army (ERP), several dozen uniformed, well-armed, masked men and women appeared at a public rally in the lush hills above Acapulco, fired volleys in the air and called for the overthrow of the government.
Later on Friday night, about 20 armed men in similar garb clashed with police at a roadblock not far away, wounding three policemen with fire from AK-47 assault rifles before melting into thick foliage.
The group's emergence shocked Mexico, where masked Indian peasants calling themselves Zapatistas and led by a pipe-smoking intellectual in a black balaclava - the now-legendary Subcomandante Marcos - rose against the government in the south-eastern state of Chiapas in January 1994.
The ERP in Guerrero said it had no connection with the Zapatistas. But its proclaimed aims sounded similar and Mexican commentators said the group, while posing no direct military threat to the Mexican army, could be a serious destabilising force for a government whose popularity is sliding.
The Zapatistas have not fought with the Mexican army since the rising but have remained a thorn in its flesh and have negotiated serious political concessions.
Some analysts felt it was no coincidence that the guerrillas in Guerrero emerged minutes after a speech by the leader of Mexico's left-wing parties, Cuauhtemoc Cardenas. Mr Cardenas later denounced them. But though he criticises violence, the son of revered former President Lazaro Cardenas considers himself a kind of "spiritual leader" of Mexico's left-wing factions and has visited Subcomandante Marcos's jungle hideout in Chiapas.
Other commentators noted that the new guerrillas were suspiciously well- armed and equipped for a peasant movement, far more so than the ragtag Zapatistas in Chiapas, and suggested they may have been concocted by one or other political faction - even, perhaps, one within the ruling Institutional Revolutionary Party (PRI) - as a lever of power.
Friday's incident occurred at a memorial service for 17 peasants massacred by Guerrero state troopers near the village of Aguas Blancas a year ago. The peasants, riding two cattle trucks, had been on their way to a left- wing rally.
The Guerrero state governor Ruben Figueroa resigned after being found responsible for the ambush and for putting pistols in the hands of the victims in an attempt at a cover-up.
On Friday, the 50 men and 10 women in crisp olive uniforms and carrying AK-47s or AR-15 rifles emerged from the hills in military formation, laid a wreath to the 17 victims and fired 17 shots in the air. They were wearing red-and-black ERP shoulder insignias and covered their faces with black bandanas tucked beneath black baseball caps.
Reading a manifesto in both Spanish and the local Indian language of Nahuatl, their leader, "Commander Ignacio", said President Ernesto Zedillo's government was "illegitimate, anti-popular, anti-democratic and sustained fundamentally by the arms of the military and police". Mr Zedillo's PRI has ruled for nearly seven decades, traditionally using fraud to keep growing opposition at bay.
"Moved by the unjust conditions of our life and work, we want a democratic and revolutionary transformation of our homeland," the guerrillas said before disappearing into thick hillside forest after 20 minutes. They were later sought by jeep loads of heavily-armed soldiers in torrential rain driven.