Hundreds of thousands of Obrador supporters packed Mexico City to protest against the result of the July 2 poll.
"I propose acts of civil resistance this week," Mr Obrador told the roaring crowd.
"Citizen committees will determine which actions under which circumstances are practical," he said. But he did not specify what actions he had in mind.
Police estimated the crowd at about 800,000 as Mr Obrador called the rally the "largest ever assembled in Mexican history."
Supporters had travelled to the Mexican capital from across the country to attend the protest and thousands had camped out over the weekend in the city's historic center.
Mr Obrador narrowly lost the vote to conservative candidate Felipe Calderon. Mr Obrador has alleged election fraud and demanded a recount.
"Fully repudiate the election fraud," Mr Obrador's followers chanted in support of the candidate, who lost by about 244,000 votes or a little more than half a percent of the 41.7 million ballots cast.
Mr Obrador launched an official challenge to the vote tally one week
ago, submitting videos and other documents to the Federal Election Tribunal.
He claims the videos prove ballot-box stuffing and other cheating that he claims swung the election to his opponent.
Earlier police estimates had put the strength of Sunday's protests at around 320,000 in the Zocalo square of Mexico City.
Mr Obrador is the former mayor of the city that is still run by his partisans.
The marchers chanted "No to another '88", evoking the fraud-marred 1988 presidential election in which leftist Cuauhtemoc Cardenas lost to Carlos Salinas.
Having agreed to hear Mr Obrador's case, the election tribunal now has until September 6 to issue a final ruling on some 220 cases his leftist Party of the Democratic Revolution has submitted.
UN and EU observers pronounced the election free and fair.
Mr Obrador called for another demonstration in two weeks on July 30, promising to double the number of protesters. "We will use civil resistance to defend democracy," he said.
Meanwhile Mr Calderon is due to begin his six-year term as President on December 1.