Updated

Mexico's leftist candidate has settled in for a long battle, promising Sunday to maintain the protest camps that have paralyzed downtown Mexico City until at least September, when the nation's top electoral court must resolve the disputed presidential election.

"We could be here for years if the circumstances demand it," Andres Manuel Lopez Obrador told tens of thousands of supporters at what has become a weekly Sunday rally in the capital's main square, known as the Zocalo.

The camps have brought Mexico City's cultural and financial center to a near standstill, forcing workers to hike to their jobs and causing traffic jams across the city for the last two weeks. The protesters are angry at an official but still uncertified vote count that gave the conservative ruling party candidate Felipe Calderon a slight lead in the July 2 election.

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Lopez Obrador has demanded a full recount, and said Sunday he still won't accept anything less.

"There's one decision that we've made from the start," he said. "We will not allow the imposition of an illegal, illegitimate government."

Lopez Obrador called for more large-scale protests on Sept. 1, when President Vicente Fox is to give his last state of the nation address, and on Sept. 6, the deadline for the court to name a president-elect. He also said he wanted to hold a "national democratic convention" on Sept. 16, Mexico's Independence Day, to "reform" the country's electoral institutions.

"I will carry on until the bitter end," he said

Standing with her two young children in the Zocalo, Maria Sanchez, from the Milpa Alta farming district just outside Mexico City, said she was ready to protest "until they give us an answer to our problems."

"We are here to say no to electoral fraud," she said. "They have to respect the will of the people. It is not fair that the rich keep getting richer and the poor keep getting poorer."

On Saturday, one of Lopez Obrador's top aides, Claudia Sheinbaum, told The Associated Press that electoral officials should annul the results from about 4 percent of the 130,000 polling places — mostly in places where Calderon won — because of "irregularities." She said that would "change the balance of the election" and make Lopez Obrador the winner.

The partial recount of 9 percent of the polling places ordered by the Federal Electoral Tribunal was to end Sunday, and the court's seven magistrates will then review the results and release them later this month.

Local media reported that variations of only a few thousand votes have been found, far short of what Lopez Obrador would need to overcome a 244,000-vote deficit.

Party representatives observing the recount say electoral officials have found extra ballots in some ballot boxes, and in other cases have failed to account for all blank ballots distributed to polling places. Lopez Obrador says that proves fraud led to Calderon's advantage of less than 0.6 percent of the vote.

Representatives of Calderon's conservative National Action Party have insisted that no major problems or variations in the vote have surfaced during the partial recount. Calderon's supporters have launched counter-protests, asking Mexicans to wear white T-shirts on Sunday to show they don't agree with Lopez Obrador.