Mexican President Vicente Fox has ordered a state governor to stop clashes between rival political parties which have left two men dead.
Fox lamented the violence in an electoral campaign
The fighting happened in the southern state of Oaxaca ahead of key governorship elections on Sunday.
It is the first time people have died in electoral-related violence in Mexico in years - and there are fears of further clashes in the remaining days.
Some of Mr Fox's opponents have said it is up to him to guarantee security.
"I truly and deeply lament the death of two persons," Mr Fox said.
"I regret very much this violence in an electoral campaign."
Mr Fox told Oaxaca Governor Jose Murat to act.
"It is very important that the state government instils order, secures the peace during this election, because electoral processes are the political responsibility of the governor of the state himself," said Mr Fox.
Tensions are high in Oaxaca in the run-up to Sunday's elections to choose a new governor.
The fatal confrontation on Tuesday appears to have happened as supporters of the Institutional Revolutionary Party (PRI) marched into town with their candidate, Ulises Ruiz.
Supporters of Mr Ruiz's key opponent in the elections, Gabino Cue - who represents the Todos Somos Oaxaca coalition - set up roadblocks to try to prevent the march, reports suggested.
One man died instantly of head wounds in the ensuring clash - and was pictured in the next day's edition of the national newspaper Reforma being beaten by two men with sticks.
Another man - apparently from the other side - was confirmed brain-dead on Wednesday.
Seven other people are reported injured.
Both parties insisted its own supporters were not in the wrong.
The clash has sparked fears among commentators that Mexico could be seeing a return to the political violence of the years running up to 2000, when the PRI lost the presidency after an unbroken spell of 71 years in power.
They have also cautioned that it could bode ill for presidential elections in 2006.