MEXICO CITY — One of Mexico’s best-known journalists, who exposed a possible conflict of interest involving the first lady’s purchase of a house from a government contractor, was fired on Sunday in a dispute that defenders see as an act of retaliation and an attack on press freedom.

The journalist, Carmen Aristegui, who has a long record of exposing the foibles of the elite in an often brusque, opinionated manner — as well as clashing with her bosses — was dismissed from MVS Radio, where she hosted one of the country’s most popular morning news and talk radio shows.

Officials at the station said she was fired after she threatened to quit if they did not reinstate two members of her investigative team. They were fired last week for using the station’s brand in promoting a new web initiative, Mexicoleaks, to collect tips and leaks about government and political corruption.

The site, which began operating last week, has reported receiving several tips.

Ms. Aristegui, who still has a daily talk show on CNN Español, vowed to challenge her dismissal, and later in the evening hundreds of supporters attended a rally at the station. She has characterized her confrontation with the station as a battle over freedom of expression, and many Mexican journalists and intellectuals rushed to defend her on social media.