Leftist Andres Manuel Lopez Obrador, who is challenging electoral results that gave victory to his conservative rival Felipe Calderon, led the protest from a yellow canvas tent on the downtown Zocalo square, facing the presidential palace.

He and his supporters spent the night camping out in a protest the politician said would continue until authorities agree to a recount of the votes cast in what he called a fraudulent election.

Many of the protesters pitched their tents along Reforma avenue, one of the city's main arteries, erecting an eight-kilometer (five-mile) blockade that caused massive traffic jams and turned parts of the megalopolis into pedestrian zones.

"Traffic has been paralyzed by the blockades, there are traffic jams in the entire city," said Fernando Puga, of the city's traffic support services.

Lopez Obrador, a fiery former Mexico City mayor, had called for the campaign during a massive rally in the capital on Sunday, which city authorities said brought together a record 1.2 million people.

He asked his backers to set up 47 sites across the city's main thoroughfares.

"I suggest we stay here, in a permanent assembly, until the resolution of the court," Lopez Obrador said during Sunday's protest rally, the third since the election.

Electoral authorities have said that Calderon got the most votes in the July 2 presidential election, but Lopez Obrador, who trailed him by just 0.58 percentage points, challenged the outcome before the Federal Electoral Tribunal.

The tribunal, the ultimate arbiter in electoral disputes, has until September 6 to formally announce the name of the president-elect, who will take office on December 1.

Earlier this month, Lopez Obrador's leftist Party of the Democratic Revolution (PRD) sent an 836-page document to the court claiming that the vote was invalid, and suppled videotapes and other alleged evidence of cheating.

Lopez Obrador wants a recount of the 41 million ballots cast in the presidential elections, claiming tallies from 72,197 polling stations, out of a total 130,500, contain errors that clearly demonstrate the vote was fraudulent.

But Calderon insisted his electoral victory was irreversible, and his conservative National Action Party (PAN) branded Lopez Obrador's statements as "schizophrenic."

The ruling PAN harshly criticized the latest protests, with spokesman Cesar Nava saying they had become "acts of aggression."

Nava insisted the blockades violated a rule set up by Lopez Obrador himself when he was mayor, which prohibits blocking the city's major arteries. He also accused the current mayor, who is also from the PRD, of spending more time supporting the leftist candidate than running the city.

The city's chamber of commerce warned the blockades affected tourism as well as the financial sector and would lead to losses of up to 36 million dollars a day, and threaten the jobs of thousands of people.

"What worries us most is that this could take a violent turn," Lorenzo Ysasi, who heads the chamber, told AFP.

But the office of President Vicente Fox insisted the protest would have no impact on the country's buoyant economy, which is forecast to grow by over four percent this year.