"This is an unjustifiable massacre, someone has to do something." said Patricia Marina, of the Fray Bartolome de las Casas Center for Human Rights.
Witnesses said paramilitaries attacked the Indians in Acteal, in the mountainous municipality of Chenalho, 44 miles (70km) north-east of San Cristobal de las Casas.
Chiapas state prosecutor Marco Antonio Bezares said on national radio that the events were tragic. "It is believed we are talking about a number above 40," he said. Manuel Perez Vazquez, a local indigenous leader, said there were 43 dead and many injured after the paramilitaries opened fire. And Domingo Perez Pacencia, president of the municipal council of Chenalho, said: "This is the worst massacre that has occurred in Chiapas since the uprising of 1994."
Some reports said the victims were supporters of Chiapas's Zapatista rebels, who burst onto the national stage on 1 January 1994 with a violent uprising against the Mexican government in which officially 140 people died.
Waves of paramilitary violence have swept the state since then as gunmen accused of being backed by landowners and local politicians have sought to punish villagers for their support of the Zapatista National Liberation Army (EZLN). Many Indians have fled their villages for the northern Chiapas highlands. Some reports said Monday's victims had already fled paramilitaries elsewhere in the region.
Hospitals as far away as San Cristobal de las Casas were overwhelmed as doctors struggled to cope with the flood of injured.