Millions of Mexicans are deciding who will succeed President Vicente Fox, as well as electing senators and deputies for the nation's two-house Congress.  Election officials say preliminary results could be available within five hours after the polls close.

The voting day began with a formal session of Mexico's Federal Electoral Institute, known by its Spanish initials, IFE. IFE President Luis Carlos Ugalde declared that all was in place for what he called a historic moment in Mexican history.

He said around 130,000 polling places have been established, and, for the first time, Mexicans living in foreign countries have been able to cast votes after registering with consulates.

Ugalde called on the news media and leaders of Mexican political parties to respect the law and refrain from announcing any results until 8:00 PM Mexico City time , when the IFE will begin releasing information from its official count in various parts of the nation. Ugalde said the electoral body will announce a preliminary tendency in the presidential race, based on a quick count using a random sample from various polling stations, at 11:00 PM local time.

But the official final result could take much longer. The presidential race remained very close when the last public opinion polls were released several days ago.

Some polls give a slight lead to former Mexico City mayor Andres Manual Lopez Obrador, the candidate of the leftist Party of the Democratic Revolution, but other polls show him in a statistical dead heat with Felipe Calderon, former energy minister and candidate of the ruling National Action Party. The candidate of the Institutional Revolutionary Party, Roberto Madrazo has been in a distant third place since the race began last year. His party ruled Mexico uninterrupted for 71 years, until the election of President Vicente Fox in 2000.

By law, Fox is limited to one six-year term. His term has been marked by an unprecedented openness in government, and efforts to strengthen the nation's trade ties with the United States and other parts of the world. However, Mr. Fox failed to get his major reform proposals through the Mexican Congress, which was divided among the three major parties.

Political analysts say the same situation is likely to prevail for whoever succeeds Mr. Fox. Polling indicates that no party will enjoy a majority in the Congress.