Human rights officials say paramilitary groups, armed and backed by the military and police, have assassinated an average five peasants a day over the last few months in the south-eastern state of Chiapas, where the Zapatista rebels rose up in 1994 and have widespread support.
Peasants have also been thrown off their land, detained and tortured if they are suspected of even sympathising with the Zapatista National Liberation Army (EZLN), the officials say.
The incidents have turned Chiapas, particularly the northern part of the state, into a powder keg. Locals talk of the potential for civil conflict in the run-up to nationwide legislative elections in July. Communities are split between support for the ruling Institutional Revolutionary Party (PRI) of President Ernesto Zedillo and the centre-left Party of the Democratic Revolution (PRD), which backs the social aims of the Zapatista rebels.
The presence of tens of thousands of troops in the state has added to the tension.
In the community of Agua Blanca, troops and police recently arrived by helicopter to harass women working in coffee plantations, according to one eyewitness. The security forces, known to locals as the BOM (an acronym for Joint Operation Force), said they were looking for weapons. When they found none, they forced the women to pose with the army or police rifles, the witness said.
It was not clear whether the BOM detained any of the women or were simply threatening them with future arrest if they had anything to do with the Zapatistas.
"The frontline war lasted only 12 days with the Zapatista uprising of January 1994," said Pablo Romo of the Fray Bartolome Centre, a leading independent human rights group which defends Chiapas's native Mayan Indians. "Now, it is a secret war of low intensity, a psychological war. There have been bombs, kidnappings and shootings aimed at suspected EZLN sympathisers, human rights groups or social workers."
At least 16 firebombs or molotov cocktails have been thrown at the offices of human rights groups in San Christobal over the past five weeks. A leading human rights activist, Javier Lopez Montoya, was kidnapped with his wife and children last November and has not been seen since.
A colleague of Mr Romo at the Fray Bartolome Centre, Jose Montero, was wounded in the arm when gunmen in civilian clothes opened fire on his car last month. Mr Montero said he saw uniformed police with the gunmen.
Mr Lomo and Mr Montero say they are in no doubt that the gunmen belonged to the so-called Paz y Justicia (Peace and Justice) group which they describe as paramilitary. The group is made up of hardline PRI supporters, led by the party's legislator in Chiapas, Samuel Sanchez, and reportedly armed and backed by the army and police.
Ex-members of Peace and Justice were quoted here this week as saying they had been given military training and were told to "do away with the Zapatistas".
Peace talks between the Mexican government and the EZLN have been stalled since last September. The guerrilla leader Subcomandante Marcos, known for his black balaclava and pipe, remains in hiding in the Lacandon jungle east of here with several hundred men. His supporters believe he is planning a new operation, more likely to be a propaganda coup than a military assault.
While the leader himself has not been seen for weeks, his image is everywhere in San Cristobal. You can buy a Marcos T-shirt for pounds 4, a Marcos balaclava for pounds 1, a Marcos-doll key ring for 40p or a Marcos clock for pounds 5.
And on the edge of the jungle, his men still rule the roost although they are blocked by the army from moving west. One of Marcos's top aides, Comandante Tacho, organised a culture show in rebel-controlled territory this week which featured the United States folk-rock group The Indigo Girls and a group of American Indians who performed a fire dance in support of the Zapatistas.