Mexico City Mayor Andres Manuel Lopez Obrador has returned to work for the first time since Congress stripped him of immunity from prosecution.
People travelled from all over the country to attend Sunday's rally
Mr Lopez Obrador vowed to return after hundreds of thousands of his supporters marched in the city on Sunday.
They crammed into a central square to oppose the decision by Congress three weeks ago that has left the mayor facing prosecution in a land dispute.
He was tipped to become president in 2006, but may now be ineligible to run.
There is also confusion over whether he is legally entitled to continue working as mayor.
Hundreds of his supporters turned out before dawn to greet him on his first day back at work.
Mr Lopez Obrador pledged to get on with his job of running one of the largest cities in the world.
He has always claimed he is a victim of a political plot to stop him from running for president next year.
But he said: "I am ready for dialogue, I am ready for meetings so that there is unity and conciliation for the good of the country."
Since he took office in December 2000, Mr Lopez Obrador - who belongs to the left-wing Party of the Democratic Revolution (PRD) - has become one of the country's most popular politicians.
Sunday's protest was one of the biggest marches Mexico had ever seen, with thousands travelling from all over the country to attend.
Organisers said 1.2 million people were present, though officials put the number at 120,000.
Mr Lopez Obrador 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.
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.
But, amid all the uncertainty, it is clear that the controversy has boosted Mr Lopez Obrador's support and cemented his popularity, says the BBC's Claire Marshall in Mexico City.