Both leading candidates in Mexico's presidential elections have claimed victory, although officials say the poll is too close to call.
Supporters from both sides gathered to await the results
A quick vote count based on sample figures had been expected to generate a result within hours of polls closing.
But with the leftist and a conservative candidates running almost neck and neck, officials have decided to count all votes before declaring the winner.
Election officials said a winner would not be announced before Wednesday.
Leftist Andres Manuel Lopez Obrador and conservative Felipe Calderon said they would respect official results.
But both gave rousing speeches to supporters gathered at their respective headquarters.
"According to our data we have won the vote by at least 500,000 votes. This is irreversible," said Mr Lopez Obrador.
Thousands of his supporters gathered in the rain in the city's central square shouted "Fraud! Fraud!", the Associated Press news agency reported.
Shortly afterwards, Mr Calderon of the governing National Action Party (PAN) party, pointed to several projections that showed him leading and announced: "We have won the presidential elections."
Street celebrations broke out in both camps after the announcements.
Amid concerns that the uncertainty could spark violence and political chaos, President Vicente Fox appealed for calm:
"Citizens, we can have complete confidence that each one of our votes will be properly counted and respected," he said.
The outcome of the vote will be watched closely throughout the Americas but is of special concern to the US, where migration from Mexico is a key issue.
Chamber of deputies: 500 seats
National Action Party (PAN): 34.2%
Alliance for the Good of All (PRD): 29.3%
Institutional Revolutionary Party (PRI): 26.6%
New Alliance: 4.9%
Social-Democratic and Rural Alternative Party: 2.3%
Senate: 128 seats
National Action Party (PAN): 34.5%
Alliance for the Good of All (PRD): 30.1%
Institutional Revolutionary Party (PRI): 26.4%
New Alliance: 4.3%
Social-Democratic and Rural Alternative Party: 2.1%
Figures with 80% of polling stations reporting
Officials say turnout was high, after a lengthy campaign marked by political insults and accompanied by a rise in drug violence.
The leading candidates have been offering alternative visions of how to create jobs and boost the economy.
Among the 70 million people eligible to vote are thousands in the US, after a decision to allow Mexicans living abroad to vote for the first time.
In congressional elections, with 80% of polling stations reporting, the PAN was in line to win the most seats but to fall well short of gaining a majority in either house.
The Institutional Revolutionary Party (PRI), which ruled Mexico for 71 years, was set to fall into third place in Congress for the first time.
The BBC's Duncan Kennedy in Mexico City says the election has been portrayed as a fight between left and right, a choice between more state intervention or the continuation of free market policies.
Mr Lopez Obrador is a former mayor of Mexico City, who has championed the poor and promised major public building programmes.
Andres Manuel Lopez Obrador - Alliance for the Good of All (PRD)
Felipe Calderon - National Action Party (PAN)
Roberto Madrazo, Institutional Revolutionary Party (PRI)
Patricia Mercado - Social Democrat and Peasant Alternative Party
Roberto Campa - New Alliance
His main rival, Mr Calderon, is a Harvard-educated former government minister who calls for more foreign investment and greater links to the global economy.
A third candidate, the PRI's Roberto Madrazo, is said to have been trailing in the polls.