Mr. Lopez Obrador made the announcement Thursday before thousands of supporters in Mexico City.

Congress is expected to vote later Thursday on whether to strip the mayor of his immunity so he can face criminal charges in a land-use case.

Mr. Lopez Obrador is accused of contempt of court for allegedly refusing a court order related to a land dispute between the city and a private property owner. He has denied any wrongdoing.

If convicted, the mayor could be jailed and barred from a presidential run.