With 100,000 people chanting "revolution," Mexican leftist Andres Manuel Lopez Obrador urged supporters Sunday to consider forming a new party and peacefully defy president-elect Enrique Pena Nieto.

Addressing a rally in Mexico City's main square, Lopez Obrador again refused to recognize Pena Nieto as the winner of the July 1 election after the electoral tribunal dismissed his bid to overturn the results over vote-buying claims.

"There is still no democracy in Mexico," said the former Mexico City mayor. "I will not recognize Pena Nieto as the legitimate president of Mexico."

Pena Nieto, whose Institutional Revolutionary Party ruled Mexico for most of the 20th century, won 38.2 percent of the vote compared to 31.6 percent for Lopez Obrador -- a margin of three million votes.

But Lopez Obrador claims that the PRI, which governed Mexico with an iron grip from 1929 to 2000, bought five million votes and violated campaign spending rules in order to secure Pena Nieto's victory.

Pena Nieto has denied the allegations and called for national unity following the court's decision.

Lopez Obrador led mass protests that paralyzed Mexico City in 2006 after he lost that year's election to Felipe Calderon, of the conservative National Action Party (PAN), by less than one percentage point. The leftist leader has never recognized Calderon as the legitimate president of Mexico.

The size of Sunday's protest paled in comparison to the massive demonstrations Lopez Obrador led six years ago.

His Movement of National Regeneration (MORENA) called for peaceful acts of civil disobedience, including protests in squares across the country on December 1, the day Pena Nieto takes office for a six-year term.

"Lopez Obrador has reminded us that we cannot be cowards when something is imposed on us," said Laura Salazar, a 34-year-old engineer. "We fought for years for the PRI to be kicked out of power. Now we must do it again."

Lopez Obrador told the crowd he was leaving the leftist Party of the Democratic Revolution (PRD), stressing that he was splitting on good terms with the party that backed his 2006 and 2012 election bids.

He called on his followers to decide during popular assemblies throughout the country starting on September 12 whether MORENA should become a political party or remain a civil society association.

"I will dedicate all my work and imagination to the cause of transforming Mexico, and I will do it through MORENA," Lopez Obrador said.

"We must try change through peaceful and electoral ways, respecting other points of view. But I don't consider violence an alternative," he added.

"Instead of destroying an authoritarian regime, violence perpetuates it, and we don't want this."

The former mayor's decision to quit the PRD will further weaken the Mexican left, said Octavio Islas, political analyst at the Monterrey Institute of Technology and Higher Education.

The rupture was long expected and reflects a "tribal war" within the left, with all eyes now on the next move of current Mexico City Mayor Marcelo Ebrard of the PRD, Islas said.

"We will see what Marcelo Ebrard will do. Many see him as the face of a modern left," he added.