Salvador Sanchez of the Farabundo Marti National Liberation Front (FMLN) told hundreds of protesters in the capital that sending scores of soldiers to Iraq was a "serious error".
"This government is responsible if anything happens to any citizen," he said.
Numerous Iraqi resistance groups have already issued a series of threats against El Salvador unless it pulled out of Iraq altogether.
But a new contingent of 380 Salvadoran soldiers is scheduled to join more than 300 compatriots already in Iraq with the US-led forces, within days.
Schafik Handal, who ran unsuccessfully as a presidential candidate in 2003, also addressed the crowd.
He said the invasion and occupation of Iraq was wrong. "War is a form of terrorism and this is a totally unjust war."
The latest threat against the new deployment came from a group calling itself the Muhammad Atta Brigades - which posted an internet statement giving the central American country 20 days to pull out of Iraq.
"This is the last chance after which there will not be any more statements, only bloodshed," said a statement carried on a website that has previously carried similar threats.
El Salvador, which has deployed more than 300 troops in Iraq since August last year, has stepped up security but said it would go ahead with the additional troop deployment, planned for 17 August.
It is the only Central American country left with troops in Iraq, after the pullout of Nicaragua and Honduras along with the Dominican Republic, all of which served under Spanish command there. Spain, too, has withdrawn its troops from Iraq.