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.
The dispute raises the prospect of legal challenges. However it is resolved, the victor will have a weak mandate to govern.
The election tribunal has not formally declared Sanchez Ceren the winner, saying it needs to review challenges to some ballots and that it is waiting for the final vote count.
However, Eugenio Chicas, head of the tribunal, 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."
He said definitive results would be ready in three or four days.
(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 Mohammad Zargham)