Hundreds of thousands of people have marched through Mexico City in support of the capital's embattled mayor, Andres Manuel Lopez Obrador.
Mr Lopez Obrador is in legal limbo as he awaits the outcome
They crammed into the city's central square to oppose a decision by Mexico's Congress that has left the mayor facing prosecution in a land dispute.
Mr Lopez Obrador was tipped to win the 2006 presidential election before Congress ended his legal immunity.
His supporters say the decision was a political move to block his candidacy.
Since he took office as Mexico City mayor in December 2000, Mr Lopez Obrador has become one of the country's most popular politicians.
He is accused of breaching a court order after allowing the construction of an access road to a city hospital said to go through a disputed plot of expropriated land.
Mr Lopez Obrador, who belongs to the left-wing Party of the Democratic Revolution (PRD), says the attempt to convict him for contempt of court is frivolous.
The numbers who joined Sunday's protest exceeded even the most optimistic expectations of the march organisers, says the BBC's Claire Marshall in Mexico City.
Many of them waved yellow balloons and wore yellow T-shirts - the colour of Mr Lopez Obrador's political party.
One group carried a coffin bearing the inscription "They want to kill our democracy, but our hope will never die."
The mayor addressed the crowd, accusing his opponents of "irrational and cowardly fear".
"We are sure our cause is just and that it will triumph," he said.
Mr Lopez Obrador's supporters cried foul after Mexico's two biggest political parties - the president's National Action Party (PAN) and the main opposition Institutional Revolutionary Party (PRI) - joined forces in Congress earlier this month to strip the mayor of his immunity.
It is now up to a judge to decide whether he should be prosecuted. If he is ordered to face trial, he will be barred under the constitution from running for office and will have to stand down as mayor.