Mexican voters started casting their ballots Sunday morning to choose the country's next president, 500 deputies and 128 senators in the presidential and congressional elections.

The elections are considered the largest in the history of Mexico, with 2,127 positions at stake, including the president, senators and deputies of the bicameral Congress, six governors and the mayor of Mexico City.

The election authority has installed 143,151 boxes across the country and printed 82.68 million ballots for the presidential election, and the same number of ballots for the voting of senators and federal deputies.

Polling stations opened at 8:00 a.m. local time (1300 GMT) and will close at 6:00 p.m.(2300 GMT) Sunday. The first national exit polls are expected when voting ends in the westernmost part of the country 12 hours later.

Mexico's Federal Electoral Institute's (IFE) President Leonardo Valdes called on citizens to vote and candidates to respect the results of the election, vowing efforts would be made to ensure that the election be conducted in a manner of "impartiality, fairness and peace."

IFE is expected to announce the quick count results of polling stations at 11:45 p.m. (0445 GMT Monday), but the official count will continue until next week.

Four candidates are competing for the presidency in a first-past-the-post system -- the one who garners the largest number of votes wins. There is no second round of voting and no minimum percentage required for an outright win.

The candidate of the Institutional Revolutionary Party, Enrique Pena Nieto, leads in the latest poll with 41 percent support, followed by Andres Manuel Lopez Obrador of the Democratic Revolution Party with 31 percent, and Josefina Vazquez Mota of the ruling National Action Party with 24 percent.

Under Mexico's constitution, the president is elected for a six-year term and cannot be re-elected. The new president will take office on Dec. 1.

Mexico's legislative power is vested in the bicameral National Congress. The Senate has 128 members, with four from each state and the Federal District, and each senator having a tenure of six years, while the Federal Chamber of Deputies, directly elected every three years, has 500 seats.