Sunday's presidential election was seen as a neck-and-neck race between Harvard-educated conservative Felipe Calderon, of the ruling party, and former Mexico City mayor Andres Manuel Lopez Obrador of the leftist Party of the Democratic Revolution (PRD.)
Later, electoral authorities said the election was too close to call and results would only be announced on Wednesday.
Third place in opinion polls went to Roberto Madrazo, the leader of the Institutional Revolutionary Party (PRI,) which ruled Mexico with an iron fist for 71 years before the historic 2000 electoral victory by Vicente Fox, of the National Action Party (PAN.)
The three parties also waged a close race in the congressional election, but none of them appeared likely to win an outright majority in the House or the Senate.
The US administration kept a close eye on the election, evidently hoping to see a reversal of Latin America's swing to the left.
The Calderon campaign has capitalised on fears among Mexican business leaders, likening his opponent to Venezuela's virulently anti-US President Hugo Chavez, and warning he would plunge the country into economic crisis.
Analysts generally dismiss the comparison and say that while Mr Lopez Obrador has little interest in foreign policy, he is not hostile to the United States, Mexico's largest trading partner.
Voter Eduardo Matias Lopez, 50, who fled Cuba 20 years ago and has since become a Mexican citizen, worried about the consequences of a Lopez Obrador victory.
"I am afraid we could have a regime like that of Venezuela or
Cuba," he said as he stood in line to cast his ballot in Mexico City.
"I fled Cuba because of an authoritarian regime and I will flee again if there is a similar regime in Mexico."
Mr Lopez Obrador rejects suggestions his policies would be anything less than democratic, and insists that the reforms he plans would improve the lot of downtrodden.
"If Lopez Obrador wins, there will be social change. It won't be a radical change, but the situation will improve, particularly for the most needy," said voter Margarita Grijalba, 50.
Mr Lopez Obrador says he would finance major job-creating infrastructure projects and hand out financial aid to elderly and handicapped people.
For his part, Mr Calderon, 43, a staunchly conservative former energy minister, wants to encourage foreign investment and slash corporate taxes in order to boost economic growth.
Mr Lopez Obrador insists the wealth is not trickling down to the millions of Mexicans who live on less than two dollars a day.
Mexico's next president, who will take office on December 1, will face formidable challenges in trying to fulfil campaign pledges of battling poverty, corruption, common crime and drug-fuelled violence.
Protest march
As voting was taking place on Sunday thousands of people, led by masked Zapatista leader "Subcommandante" Marcos, took to the streets of Mexico City claiming the elections did not offer a real solution in a country where millions live in poverty.
"Our ideas of justice and liberty are bigger than the ballot boxes," the protesters chanted as they marched to the historic downtown Zocalo square, where hundreds of people lined up to vote.
"We want to demonstrate that there is an alternative to the ballot box," said Maria Fernandez, a militant of the Zapatista movement, which rose up in arms in the impoverished southern state of Chiapas in 1994. The insurgency left 150 people dead before a ceasefire was declared.
The small guerrilla army has now become a political movement regrouping several indigenous and peasant organisations and radical leftist militants.
On January 1, Marcos led a so-called "other campaign," touring Mexico in a bid to launch a radical alternative to the leftist Party of the Democratic Revolution (PRD), which he claims has betrayed leftist ideals and does not truly represent impoverished Mexicans who make up about half the 103 million population.