By RANDAL C. ARCHIBOLDMARCH 16, 2015

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.

At least three prominent journalists said they would no longer appear on MVS Radio. One of those journalists, Denise Dresser, a widely read columnist at Reforma, one of the country’s largest dailies, said the station’s complaints about misusing its brand were “a smoke screen.”

Ms. Aristegui was briefly fired from MVS in 2011 after reporting on unsubstantiated rumors about Felipe Calderón, the president at the time. Station managers later said they had been pressured by the president’s staff.

Andrew Selee, a veteran Mexico scholar at the Wilson Center in Washington, said that while Ms. Aristegui often left little doubt about her left-leaning politics, her investigative journalism filled a void in Mexico.

“In a country where newspapers have not always done in-depth investigative reporting, like other countries, she filled that role,” Mr. Selee said. President Enrique Peña Nieto’s office would not comment on Ms. Aristegui’s work and whether it played a role in her firing.

A statement from the Interior Ministry called on Ms. Aristegui and the station to settle their differences through dialogue and asserted that “the government has respected the exercise of critical and professional journalism, and will continue doing so with the conviction that the plurality of opinions is indispensable for the strengthening of democratic life in the country.”

Ms. Aristegui and the two reporters who were dismissed, Daniel Lizárraga and Irving Huerta, first reported in November that the first lady, Angelica Rivera, was buying a house on credit from a contractor doing business with the government. She later agreed to end the deal, though the president’s office said there was no wrongdoing and no conflict of interest under Mexican law.

Ms. Aristegui and her team published the article on her news website,AristeguiNoticias.com, but their association with MVS was well-known. A former lawyer for the station, Eduardo Sánchez, was appointed the president’s communication director earlier this month.