By Duncan Kennedy
BBC News, Mexico City
Thousands of Mexicans have again turned out at a rally in support of the left-wing candidate in the contested presidential election.
Lopez Obrador accuses an elite of kidnapping the nation's institutions
Andres Manuel Lopez Obrador told supporters in Mexico City's vast main square that they had a constitutional right to try to change the government.
He claims he was denied victory in the election by systematic vote-rigging.
That is denied by the probable winner in the election, Felipe Calderon - and by Mexico's top electoral court.
Under a burning sun and festooned in their yellow party colours, thousands of supporters came again to listen to Mr Lopez Obrador speak.
Thirty-six days into their street demonstrations, the tone of defiance was as strong as ever.
Mr Lopez Obrador said a small elite had kidnapped the nation's institutions.
He said what he was doing was constitutional and that he had the right to try to change the government.
Lopez Obrador didn't mention his legislators' takeover of Congress
"We will not accept a puppet president," he said - referring to Mr Calderon, his conservative rival.
Mr Lopez Obrador made no mention of Friday's chaotic scenes in the Congress when dozens of his deputies took over the podium and prevented President Vicente Fox from making his state-of-the-nation speech.
Instead, Mr Lopez Obrador told his followers that he would go ahead and set up what he called a "national democratic convention" in two weeks' time.
He has already hinted at establishing a parallel government.
Mr Calderon has spoken of the need to bring Mexicans together. He is expected to be confirmed as the president-elect in the next three days.