Supporters of Mexico's leftist presidential candidate Andres Manuel Lopez Obrador are marching on the capital, demanding a ballot-by-ballot recount.

Hundreds of demonstrators began arriving Wednesday, gathering for a mass protest in Mexico City's Zocalo Square this Sunday.

Meanwhile, the election's apparent winner, Felipe Calderon, called for calm as demonstrators continue to challenge the results of the July second presidential election.

Lopez Obrador's supporters say widespread irregularities in the vote allowed Calderon to win the initial count by a slim margin.

Protesters say they have evidence of fraud, including videos showing a person stuffing ballots into a ballot box in the state of Guanajuato.

Lopez Obrador's staff has said street protests would continue indefinitely.  On Saturday, more than 100-thousand demonstrators gathered in the capital.

Mexico's electoral tribunal has until September sixth to rule on Lopez Obrador's challenge and officially announce Mexico's next president.

Calderon has already begun to assemble his transition staff.  On Tuesday, he named his campaign adviser, Juan Camilo Mourino, to head the team, and put Josefina Vazquez Mota in charge of relations with other parties.

Some information for this report was provided by AFP, AP and Reuters.