Tens of thousands of supporters of Mexico's leftist candidate gathered in the capital Sunday for what they hoped would become the nation's largest-ever street protest, demanding a vote-by-vote recount of an election they claim was marred by fraud.

It was the third rally convened by presidential hopeful Andres Manuel Lopez Obrador since the July 2 vote and there were concerns he might radicalize his followers, who have so far been peaceful and orderly.

Lopez Obrador's aides say they expect 1.5 million to 3 million people to attend the demonstration. While his supporters and city police have sometimes exaggerated crowd estimates, the former mayor has previously draw hundreds of thousands into the streets.

Even as demonstrators filled Mexico City's historic city center Sunday, lawyers for his conservative opponent, ex-energy secretary Felipe Calderon, were preparing to argue in front of the Federal Electoral Tribunal — Mexico's highest election court — that the election was fair and Calderon the winner. The tribunal is weighing challenges filed by both sides.

An official count gave Calderon an advantage of less than 0.6 percent over Lopez Obrador, about 240,000 votes out of 41 million-plus cast. The electoral court has until Sept. 6 to either declare a winner or annul the election. Mexico's constitution limits presidents to one, six-year term and President Vicente Fox, of Calderon's National Action Party, leaves office Dec. 1.

On Saturday, three of Lopez Obrador's lawyers asked the electoral court to declare their candidate president-elect, arguing that there were mathematical errors, falsifications or other problems at 72,000 of the country's 130,000 polling places. The closed-door session was the court's first hearing since the disputed vote.

Lopez Obrador says widespread fraud, illegal government spending to bolster Calderon and a dirty campaign run by his opponent cost him the election. His attorneys told the court that Calderon enjoyed his largest advantages at polling places where there were no observers from Lopez Obrador's Democratic Revolution Party, and that many of the votes annulled by electoral officials were in favor of Lopez Obrador.

Calderon's legal team has its chance to respond Sunday — and also plans to present its own arguments that look to annul votes in heavily Lopez Obrador districts.

Marchers gathered from across the country for Sunday's protest, riding by bus and sometimes walking from outlying states.

"They are going to have to recount the votes, because if they don't, there is going to get dangerous," said Francisco Soto Garcia, 66, a retired federal employee who set out for the march Saturday with a band of leftist supporters from the city of Taxco, in southern Guerrero state.

Soto Garcia, who walked for several miles with a band of about two dozen fellow activists before boarding a bus for the capital, said she didn't think "there will be a war, but Mexicans will stop believing in democracy here, and will never vote again."

Lopez Obrador, who stepped down as Mexico City mayor last year to run for president, hoped to fill a 5-mile stretch of the capital's main boulevard, with the march ending in the city's historic main square. Loudspeakers and giant television screens were set up along the march route, and some 2,500 city police were assigned to the event.

Lopez Obrador has regularly organized major marches.

On July 16, about 300,000 of his supporters marched to protest alleged fraud in the July 2 vote, and in April 2005 he drew between 300,000 and 400,000 to protest attempts to impeach him while he was still mayor. That 2005 march was similar in size to a silent protest in June 2004 against violent crime rates in Mexico City. Both were described at the time as the largest demonstrations in recent Mexican history.

Calderon remains confident he was the election's rightful winner, telling a group of National Action mayors at his party's headquarters on Saturday, "At the end of the day, we won the presidential election. Period."