The defeated candidate in Mexico's presidential election, Andres Manuel Lopez Obrador, has announced he is leaving his left-wing coalition to form a new political youth movement.
Speaking to tens of thousands of his supporters in Mexico City, Mr Lopez Obrador said he would focus on changing Mexico through the new group, Morena.
He said he left on good terms, after losing two presidential elections.
He refused to accept the results of July's poll, saying it was fraudulent.
Analysts say his departure from the main coalition could weaken the left in Mexico.
"This isn't a rupture," Mr Lopez Obrador said at the rally in Zocalo Square.
"I have separated from the parties that form the Progressive Movement, but I must express my deep gratitude to all party leaders and supporters."
Morena, also known as the National Regeneration Movement, has yet to be formally registered as a party.
Mr Lopez Obrador, who ran in the election for Democratic Revolution Party (PRD), also repeated his insistence that he would not recognise Enrique Pena Nieto as the legitimate president of Mexico.
He called for a campaign of "peaceful civil resistance", but stressed that protests should not turn violent, as this would only "perpetuate the regime".
After a recount of half of the vote, Mr Pena Nieto was declared the winner of July's election, with 38.2% of the vote to 31.6% of his main opponent.
Mr Lopez Obrador rejected the result, accusing Mr Pena Nieto's Institutional Revolutionary Party, or PRI, of buying votes and favourable media coverage.
But on 31 August, Mexico's Electoral Court rejected his appeal, saying there was no evidence of irregularities in the campaign or the vote.
Addressing his supporters in the capital's main square, Mr Lopez Obrador accused the court's judges of "turning a blind eye" to the irregularities in the election, describing them as "characters without conviction".
Mr Pena Nieto is due to be sworn in on 1 December for a six-year term.
The BBC's Will Grant in Mexico says that the apparently mutual decision to split with the traditional left suggests Mr Lopez Obrador does not have wide support for continuing a long-term fight against the inevitable succession of Mr Pena Nieto to the presidency.
Six years ago, after losing the presidential election by a narrow margin, he led weeks of protests that caused disruption in central areas of the capital.
This time, he says he does not want to disrupt the lives of ordinary citizens.
"We are fighting for ideals," he said. "It is a matter of honour."