MEXICO CITY — Claiming fraud robbed him of the presidency, leftist candidate Andres Manuel Lopez Obrador led hundreds of thousands of marchers through Mexico's capital Sunday, demanding a full recount in the disputed election apparently won by his conservative opponent.

The Roman Catholic Church canceled Mass at the downtown cathedral as protesters overwhelmed the massive central plaza and spilled for blocks down nearby streets. Bands played, firecrackers boomed and the leftist party's yellow banners waved in the breeze.

Police officials from the pro-Lopez Obrador city government said many as 900,000 people took part. On the ground, the crowd appeared to be much smaller, though still vast.

Lopez Obrador, a former Mexico City mayor, is demanding a full recount of the July 2 election — vote by vote, rather than relying on polling-place reports from election night, as is usual.

"To defend democracy, we are going to be begin peaceful civil resistance," a stern-faced Lopez Obrador told cheering supporters.

In official returns, Felipe Calderon of President Vicente Fox's conservative National Action Party led by about 244,000 votes — just 0.6 percentage point — though by law, he cannot be declared president-elect until a court deals with challenges to the election.

Lopez Obrador's Democratic Revolution Party has appealed to overturn the official count, alleging illicit government and corporate help for Calderon, ballot stuffing and other irregularities.

Calderon says there is no legal basis for a complete recount. He is building a transition team and planning a nationwide victory tour.

A carnival atmosphere prevailed Sunday, with grandmothers dancing to the beat of hand-held drums, teenagers tossing firecrackers and a naked bicyclist with anti-fraud messages painted on his body weaving through the crowd.

Lopez Obrador supporters compared the vote to the fraud-stained 1988 election lost by leftist candidate Cuauhtemoc Cardenas.

"We could be here six more years," said Xochitl Luna, a 43-year-old unemployed secretary, referring to Mexico's presidential term.

"In 1988 we were ready to take up sticks and stones," she said. "Today we are prepared to fight with ideas."