MEXICO CITY, July 2 — Both leading candidates claim victory, but officials say they won't sort things out until Wednesday.

Election officials declared Sunday that they could not immediately determine a winner in the tightest presidential race in the country's history. Minutes later, the two front runners each declared victory, setting in motion an electoral crisis.

While one camp alleges fraud, the president calls for calm. Earlier in the evening, tension gripped the capital as it became clear the margin in the race was razor-thin. Mr. López Obrador did not arrive at the downtown hotel where he was expected to receive the returns, but closeted himself in his campaign headquarters instead. Mr. Calderón also remained out of sight at his party headquarters.

Luis Ugalde, the head of the Federal Electoral Institute, appeared on national television and urged candidates and their supporters to wait for official results. President Vicente Fox also addressed the nation, pleading with voters to heed the election commission's decision. "It's the responsibility of all political actors to respect the law," he said.

But Mr. López Obrador, who critics say has an authoritarian streak, acted as if he was already the president elect. He went immediately to the historic central square, where thousands of his supporters had gathered to celebrate.

"We are going to demonstrate that we won and they have to respect our victory," he told the crowd.

At stake in the contest is whether the country remains on a conservative track and stays a firm United States ally or joins a trend that has brought several leftists to power in Latin America in recent years, weakening Washington's influence.