By Nelson Renteria and Michael O'Boyle
SAN SALVADOR (Reuters) - A former Marxist rebel commander's tiny lead in El Salvador's presidential election is irreversible, the country's electoral tribunal said on Monday, but his right-wing challenger demanded a full recount, insisting he was the real winner.
Salvador Sanchez Ceren of the ruling Farabundo Marti National Liberation Front (FMLN), which as a rebel group fought a string of U.S.-backed governments in the 1980-1992 civil war, claimed victory on Sunday after preliminary results showed he had won 50.11 percent support.
Challenger Norman Quijano, a former mayor of San Salvador and candidate of the right-wing Nationalist Republican Alliance (Arena) party, had 49.89 percent support.
The two men were separated by just 6,634 votes.
Quijano said in a Tweet on Monday that he wanted a "vote-by-vote" recount, calling the election tribunal biased and insisting that he was the president-elect.
"The behavior of the Supreme Election Tribunal has never been clean; they were tainted before and during the electoral process," Quijano said.
He also asked the country's attorney general to guard all electoral material and urged international observers to remain in El Salvador a few more days.
But Eugenio Chicas, head of the tribunal, said the law did not allow for a vote-by-vote recount, but that a recount of disputed polling stations would begin Tuesday.
The election tribunal has not formally declared Sanchez Ceren the winner, pending the review of challenges to some ballots and a definitive vote count.
Chicas told reporters earlier on Monday: "We put our technical teams to work all night, which is why I can tell you with certainty that the result of this election is irreversible."
Quijano later vowed to exhaust all legal avenues to challenge the result, and called for all ballot boxes to be opened.
"We're not going to allow them to steal the election from the people," he told reporters. "Once we have gone through this process with international organizations observing, once legality is respected and a definitive result springs from that, we will respect it."
However it is resolved, the victor will have a weak mandate to govern.
The FMLN and Arena - founded by the late Roberto D'Aubuisson, who had links to death squads - were fierce enemies during the civil war that killed about 75,000 people.
Quijano accused the election tribunal of corruption and hinted at foul play. He also warned on Sunday the armed forces were watching the election process.
"We are not going to allow fraud ... We are 100 percent convinced that we have won," he said. "They are not going to steal this victory. We will fight, if necessary with our lives."
While viewed as inflammatory political rhetoric, some saw a worrying tactic to undermine the result.
"I take it seriously as a political tactic rather than as a literal call, though I was troubled by his call to the army," said Hector Perla, an expert on Central America at the University of California Santa Cruz.
"All the rhetoric indicates that they are not going to accept, even if they do have the recount, they are not going to accept this other legitimate outcome," he added.
Sanchez Ceren claimed victory after the preliminary results showed him winning and he promised to govern for workers and business leaders alike.
"We are going to govern for everyone, for those who voted for us, and those who did not," Sanchez Ceren told supporters.
A carpenter's ninth son, Sanchez Ceren was a rural school teacher before joining the FMLN guerrilla movement and he then rose to lead one of its five main factions during the war.
Along with other FMLN leaders, he has moderated his policies since the peace accords were signed in 1992 and the rebel group became a political party.
The affable, media-shy 69-year-old has said he will build on its social programs, which include a glass of milk a day for children and free school uniforms, shoes and supplies.
(Additional reporting by Noe Torres in San Salvador and Dave Graham, Miguel Gutierrez, Alexandra Alper and Julia Symmes Cobb; Writing by Simon Gardner; Editing by Kieran Murray, Ross Colvin and Lisa Shumaker)