MEXICO CITY — With only 6,634 votes separating them, both candidates in El Salvador’s presidential race proclaimed victory on Monday, revealing a country equally divided between the political heirs of forces that fought a bloody civil war a quarter of a century ago.

Although Salvador Sánchez Cerén, a former rebel commander who is the governing party’s candidate, held a narrow lead, the electoral tribunal said that the close vote made it impossible to name a winner and told the candidates not to declare victory.

It would take a more definitive count, which began Monday, to determine the eventual outcome, said Walter Araujo, a magistrate who explained that the tribunal would need to review blank and contested ballots and complete counting the 10,500 absentee ballots cast by Salvadorans living abroad.

But neither Mr. Sánchez Cerén nor his opponent, Norman Quijano, a two-term mayor of San Salvador, heeded the tribunal’s warning.