Lopez Obrador said he would present his allegations on Sunday to the nation's electoral court and request that every one of more than 41 million votes be recounted to expose fraud he believes cost him the election.
"We are going to ask that they clean up the elections. We are going to ask that they count all the votes, vote-by-vote, poll-by-poll," Lopez Obrador said to wide applause.
He also called for marches nationwide Wednesday, converging on Mexico City for another rally next Sunday. And he provoked groans of disappointment when he told them not to block highways.
"This has been and goes on being a peaceful movement," he said. "We are not going to fall for any provocations."
The massive show of defiance suggests just how difficult it will be for the ruling party's Felipe Calderon to unify Mexicans, many of whom believe the nation has yet to overcome the decades of institutional corruption and fraud that kept its leaders in power.
Today Lopez Obrador turned up the heat, calling this election the most fraudulent in decades. Worse, he told foreign reporters, than under 70 years of authoritarian one-party rule, reports CBS News correspondent Bill Whitaker.
Legal challenges were built in to Mexico's elections process in recent years to help ensure clean elections, so Calderon can't be declared president-elect until the electoral court weighs allegations of fraud or unfair campaign practices. The court has until Sept. 6 to declare a winner.
Meanwhile, both Calderon and Lopez Obrador have appealed to the court of public opinion.
The stakes are high: Lopez Obrador remains convinced he won the elections. He has millions of extremely devoted followers who believe only he can help Mexico's poor and downtrodden, and he views street protests as an effective means of pressuring the government and the courts.
Calderon says the vote was clean and has taken congratulatory phone calls from U.S. President George W. Bush and the leaders of Canada, Spain and Colombia, among others, despite Lopez Obrador's plea for foreign governments to hold off on recognizing the result.
The crowd in the Zocalo Saturday night would accept nothing less than victory for the silver-haired former Mexico City mayor.
"We are never going to recognize this man (Calderon)," said Apolinario Fernandez, 37, a teacher from Lopez Obrador's home state of Tabasco in the southeast. "If he wants, let him govern in the north for the rich, but not in the south."
Calderon's strength is in Mexico's industrialized north while most of Lopez Obrador's supporters come from Mexico City and poor southern states. Many traveled all night to arrive at the demonstration, joining a sea of yellow, the color of Lopez Obrador's Democratic Revolution Party.
Election officials say Calderon, of Fox's National Action Party, beat Lopez Obrador by less than 244,000 votes out of 41 million ballots — or a margin of about 0.6 percent.
But Lopez Obrador's supporters were not convinced.
"We are ready to do whatever is necessary," said Belasario Cruz, 32, a farmer from Tabasco. "We are tired of the rich having everything and the poor having nothing."
There were no immediate reports of arrests or violence at the protests.
Lopez Obrador told foreign correspondents earlier Saturday there were more irregularities in last Sunday's balloting than in elections under the Institutional Revolutionary Party, or PRI, which ruled Mexico for 71 years until it was ousted by President Vicente Fox in 2000.
"The National Action Party learned from the fraudulent practices of the PRI and it exceeded them," Lopez Obrador said.
Political analyst Oscar Aguilar said violent protests appear unlikely while the court considers Lopez Obrador's challenge, but predicted he'll never recognize Calderon's victory.
"He will never concede defeat," Aguilar said. "Once the election results are certified, he will open a permanent campaign of criticizing the government."
Lopez Obrador claims a manual recount would confirm that hundreds of thousands of votes for him remain uncounted, miscounted or voided. The law allows such a recount only for specific polling places where credible evidence of irregularities exist. The leftist's supporters say that applies to at least 50,000 of the approximately 130,000 polling places.
Lopez Obrador must walk a tightrope: If he appears too radical, he risks hurting his party and its chances in the next presidential elections in 2012. If he appears too moderate, he risks disappointing his core supporters.
"His political stock would increase greatly for 2012" if he finds a way to concede defeat gracefully, Aguilar said.