Supporters of the left-wing candidate in Mexico's general election, Andres Manuel Lopez Obrador, have begun converging on the capital, Mexico City.
Lopez Obrador has summoned his supporters from around the country
Rallies are expected to begin in the city's vast main square, the Zocalo, leading up to a huge march on Sunday.
Mr Lopez Obrador is demanding a manual recount of votes cast in the presidential election.
Results show his conservative opponent Felipe Calderon won by a margin of only 0.57 of a percentage point.
Carrying banners and wearing the yellow colours of Mexico's left-wing Party of the Democratic Revolution (PRD), supporters of Mr Lopez Obrador are making the long journey to the Zocalo.
Buses are leaving from 300 polling stations across Mexico, and will stop off at towns along the route to pick up more activists and hold local demonstrations.
Mr Lopez Obrador has demanded a vote-by-vote recount, and has presented some 900 pages of alleged evidence of electoral irregularities to the Federal Electoral Tribunal, which must make a ruling by 6 September.
"I won the presidential election, I am more and more sure of that," Mr Lopez Obrador, the former mayor of Mexico City, told a news conference on Wednesday, Reuters news agency reported.
Supporters of Mr Lopez Obrador joined a mass rally in Mexico City last weekend, to call for a recount.
His ruling-party rival, Felipe Calderon, has said a complete vote count would be illegal, though the law does allow for recounts at specific polling stations where irregularities are reported.
Mr Calderon, of National Action Party (Pan), has said he will respect the tribunal's decision, but in the meantime is making preparations for government.
He has named two senior aides to head his transitional team, and is planning a victory tour of Mexico.
In an interview with the Washington Post newspaper on Tuesday, Mr Calderon said he would try to avoid "the escalation of tensions" by refusing to call his supporters into the streets to counter Mr Lopez Obrador's.
But Mr Calderon said he expected Mr Lopez Obrador to continue calling for massive protests.
Electoral observers from the European Union have said they found no irregularities in the vote count.