MEXICO CITY – Hundreds of thousands of protesters marched through the Mexican capital on Sunday to demand a manual recount in the disputed presidential election, led by a leftist candidate who says fraud cost him the presidency.

As a precaution, the Roman Catholic Church canceled Mass at the city’s downtown cathedral as supporters of Andres Manuel Lopez Obrador began to overwhelm the central plaza to the sound of firecrackers and bands. Police officials in the pro-Lopez Obrador city government estimated the crowd at 900,000.

Conservative candidate Felipe Calderon, of President Vicente Fox’s National Action Party, won by about 244,000 votes in the official count after the July 2 election.

Lopez Obrador’s Democratic Revolution Party has appealed to the federal electoral court to overturn the official count, alleging illicit government and corporate help for Calderon, ballot stuffing and other irregularities. The former Mexico City mayor says he will stop the protests only if there is a ballot-by-ballot recount.

National Action has also filed its own challenges, seeking to stretch Calderon’s tiny vote advantage. Calderon has said he believes is no legal basis for a complete recount, and has called on Lopez Obrador to respect official vote tallies