Two bitter rivals declared themselves Mexico's next president and electoral officials said they wouldn't name a winner for days, sparking cries of fraud and fears of violence as Mexicans waited Monday for a tedious vote-by-vote hand count. A conservative bureaucrat and a leftist were separated by fewer than 300,000 votes with more than 30 million counted in a preliminary tally by electoral officials. With results tallied from 81.7 percent of polling places, Felipe Calderon had 37.02 percent to Andres Manuel Lopez Obrador's 35.90 percent. But the Federal Electoral Institute stressed those results weren't final _ and said it wouldn't declare a victor until an official count it wasn't even starting until Wednesday. In the meantime, both candidates declared victory, raising questions about their pledges to respect an electoral process in which Mexicans invested hundreds of millions of dollars (euros) to overcome decades of systematic fraud. "We have no doubt that we have won the presidential election," Calderon told supporters. "Smile: We've already won," Lopez Obrador told his. "We're going to defend our triumph. We aren't going to let them try to make our results disappear," Lopez Obrador assured thousands of cheering supporters gathered under a steady rain in Mexico City's enormous Zocalo plaza. "Lie! Lie! Fraud! Fraud!" the crowd chanted. A drawn-out period of uncertainty could rock financial markets and unsettle Mexico's maturing democracy. Tensions were already running high after a two-year campaign marked by vicious personal attacks. Calderon painted Lopez Obrador as a radical leftist who would ruin the economy, while Lopez Obrador called Calderon a liar who doled out million-dollar favors to a brother-in-law while serving as energy secretary. The campaign exposed Mexico's deep class divisions, with Lopez Obrador of the Democratic Revolution Party pledging to govern for the poor and Calderon, of the ruling National Action Party, seen by many as the candidate of the rich. Many feared the close result could cause the tensions to explode. "If Lopez Obrador is declared the loser and it's 4 or 5 percentage points, I think you will have very ugly demonstrations in Mexico City and Oaxaca," George Grayson, a Mexico expert at the College of William & Mary in Virginia, said before the vote. For decades, elections were rigged to ensure the ruling party's victory _ fraud that allegedly included the 1988 presidential count in which a computer crash was blamed for a stunning turnaround that ensured another six years in power for the Institutional Revolutionary Party, or PRI. Many members of Democratic Revolution regret not fighting harder to challenge the loss of leftist Cuauhtemoc Cardenas, who went on to found their party. "This is no longer the era of fraud, because the people will not accept it. It is no longer '88," Lopez Obrador said Sunday night. In part because of outrage over the 1988 elections, the PRI was defeated in 2000 after 71 years in power, and sank to a distant third Sunday. President Vicente Fox, who finishes his single six-year term in December, appealed for patience and calm, saying: "It is the responsibility of all of the political actors to follow the law and respect the time the institute needs to announce the election results." U.S. Ambassador Antonio Garza, who served as an election observer in a poor Mexico City neighborhood Sunday, said he was "convinced Mexicans will wait patiently and prudently as the Federal Electoral Institute reviews today's voting records." Some voters said they had no problem waiting _ because they were convinced the official results would confirm their candidates' victory. "Now we just have to wait for them to officially confirm Felipe's victory," said Marcela Chavez, a 25-year-old economy student and Calderon supporter. "The tendency is clear and he is going to win." In other races, National Action did well in three governors contests _ Morelos, Guanajuato and Jalisco _ while Marcelo Ebrard of Democratic Revolution easily won the Mexico City mayor's post, exit polls indicated. National Action appeared to win the most seats in both houses of Congress _ but was far from a majority in either. The PRI fell into third place in Congress for the first time. The estimated 11 million Mexicans living in the United States were allowed to vote from abroad for the first time, but the 32,632 ballots they cast weren't likely to make much of a difference. "The main thing is, the door has been opened," said Jesus Hernandez, who sent in his ballot from California. "Later, we can reconstruct the procedures to make it easier in the future."