The man who led Mexico's main leftist party in the past two presidential elections has announced he is leaving it behind and may start a new party, throwing uncertainty over the future of the nation's political left.

Andres Manuel Lopez Obrador told supporters at a rally at Mexico City's main plaza on Sunday that he is leaving the Democratic Revolution Party "on the best of terms". He also announced he is leaving the smaller Labor Party and Citizens' Movement, which also backed him in the July presidential election, when he finished second.

Lopez Obrador said he would begin consultations that would create a new party out of another, less formal organisation that backed him, the Movement for National Regeneration.

The motives for the break were not clear, but it could complicate efforts for the left to rally again around a single candidate as it has in every election since 1988.

Lopez Obrador has been the most prominent figure within the Democratic Revolution in recent years, one of only two people it has ever run for the presidency since forming in the wake of the fraud-tainted 1988 election.

Still, he has not been able to dominate the structure of the party, which has suffered through bitter internal feuds, many of which Lopez Obrador's faction has lost.

Lopez Obrador, who turns 59 in November, was one of many political figures who abandoned the ruling Institutional Revolutionary Party in 1988 to support the candidacy of Cuauhtemoc Cardenas. That alliance of populist insurgents from the ruling party with a collection of small socialist parties led to the formation of the Democratic Revolution.

Lopez Obrador was a popular Mexico City mayor before seeking the presidency in 2006. He lost that election by less than a percentage point and his supporters then staged months of disruptive street protests, alleging fraud gave the victory to National Action Party candidate Felipe Calderon.

This year Lopez Obrador finished about seven points back, losing to PRI candidate Enrique Pena Nieto. While his supporters again have alleged election irregularities, the protests have been more subdued.

Police estimated about 38,000 people were in the crowd cheering Lopez Obrador on the city's Zocalo. Some were ready to jump ship with him.

"I'll go where Andres Manuel goes," said Maria Petra, a 42-year-old housewife.

"There was the PRD until a while ago. Now, no," said another spectator, Efren Garcia, 31.