Mexico City (CNN) -- Mexicans headed to the polls to vote Sunday in what officials have called "the largest and most complex election day" in the country's history.
Four candidates are vying for the presidency. Voters will also cast ballots in congressional contests and, in six states, gubernatorial races.
"Never in Mexican democracy have so many posts been at play in the same popular election," Federal Election Institute President Leonardo Valdes said in a statement.
More than 2,100 federal, state and local offices will be decided by Sunday's vote, the institute said.
For the first time, more than 79 million people were registered to vote, according to the institute. Among them are 3.5 million young people who will vote for the first time, the institute said.
Nationwide, authorities said there would be more than 143,000 polling stations and more than 13,000 accredited observers.
In Mexico City, some polls opened late, prompting criticism from voters. Lines snaked around schools and, in some cases, entire blocks in the sprawling metropolis.
At one polling station in Mexico City, a 46-year-old attorney from the state of Michoacan was fuming. Thalia Vasquez was one of hundreds of out-of-town voters who went to a special polling place to vote and had to wait for hours.
According to Vasquez, she arrived at one poll at 4 a.m., only to be told at 8 a.m. that she needed to go to a different polling place. By the time she arrived, the polling place hadn't opened, and there were already more than 100 people waiting in line. Shouting matches ensued when people tried to cut in line, she said.
"Imagine how long this is going to take," she said, saying election authorities should do more to monitor the lines outside the polls.
Voters still in line after polls close at 6 p.m. will be allowed to vote, said Ana Isabel Fuentes, a spokeswoman for the election authority.
Election authorities said Sunday afternoon that the day was proceeding normally.
Mexicans also cast ballots from beyond the country's borders. On Saturday, election officials said they had received 40,737 absentee ballots from Mexicans living abroad.
By mid-day Sunday, all four presidential candidates -- Andres Manuel Lopez Obrador of the Democratic Revolution Party, Enrique Peña Nieto of the Institutional Revolutionary Party, Gabriel Quadri of the New Alliance and Josefina Vazquez Mota of the National Action Party -- had voted.
Voters will elect new governors in the states of Chiapas, Guanajuato, Jalisco, Morelos, Tabasco and Yucatan. In Mexico City, the nation's capital, residents will elect a new mayor.