When President Yoon Suk Yeol of South Korea shocked the nation by declaring martial law, he placed news organizations under the rule of military command and outlawed “fake news.” It was a striking escalation of his long-running feud with media critical of his administration.

But when faced with censorship by the military, the Korean press did not acquiesce. News organizations spanning the political spectrum — even right-leaning publications more aligned with Mr. Yoon’s conservative People Power Party — stood united in criticism of his actions and any efforts to limit a free press.

Mr. Yoon, a former prosecutor, described his decision to declare martial law late on Tuesday as an act “of national resolve against the anti-state forces that are trying to paralyze the essential functions of the state and disrupt the constitutional order of our liberal democracy.” As part of the declaration, the South Korean military issued a decree prohibiting “fake news, public opinion manipulation, and false propaganda,” placing all media and publications under its control.

While Mr. Yoon reversed the declaration about six hours later, it offered a glimpse of the risks posed by years of challenges to press freedoms in South Korea by the country’s political leaders.

An editorial in Chosun Ilbo, one of South Korea’s biggest daily newspapers with a conservative leaning, called the president’s actions an international “embarrassment.” Mr. Yoon needed to answer to the public on how he intended to “take responsibility” for this situation, it added.

“How can the current situation justify restricting the basic rights of citizens?,” the paper wrote.

In the liberal Hankyoreh, an editorial declared: “The Republic of Korea’s biggest security risk is ‘Yoon Suk Yeol.’”