This small but densely populated Central American country of six million is plagued by brazen gang violence and still burdened by the legacy of its bitter 1979-1992 civil war.
Amid tight security, some 4.9 million voters were called to choose a successor to President Mauricio Funes of the leftist Farabundo Marti National Liberation Front (FMLN).
While there are five candidates, two are far ahead in pre-election surveys.
The top contender is Funes's vice president, Salvador Sanchez Ceren, a former civil war guerrilla commander who on yesterday promised an inclusive government if he wins.
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The 69-year-old added he would be "open to the participation of different sectors" and open the doors to all candidates to work together for "a grand national accord."
His main rival, ex-San Salvador mayor Norman Quijano, 67, of the conservative Nationalist Republican Alliance (ARENA), voted at a school in the west of the city, praising what he called "peaceful elections."
However, his candidacy is being overshadowed by corruption allegations against an ARENA campaign adviser, ex-president Francisco Flores (1999-2009).
The latest polls predict that Sanchez Ceren, a former teacher, will garner between 38 and 47 percent of votes -- not enough to guarantee a first round victory.
If no candidate gets at least 50 per cent, the top two will meet again in a runoff scheduled for March 9.
In an apparent quest to get out the vote, supporters of the various candidates set up stands in their party's colors across the capital. Funes also urged his compatriots to cast ballots.