It’s a scene that Maria Ressa, a co-founder of an online news start-up critical of President Rodrigo Duterte of the Philippines, has grown accustomed to.

Late Wednesday afternoon, several plainclothes agents from the National Bureau of Investigations arrived in Ms. Ressa’s newsroom in Manila with a warrant for her arrest in a digital libel case involving her online news site, Rappler. An hour later, they took her and her lawyer away.

Ms. Ressa’s arrest is the most dramatic sign yet of Mr. Duterte’s crackdown on the news media in the Philippines. Mr. Duterte has not tried to hide his disdain for journalists, calling reporters “sons of bitches” and “spies” and even warning that they are “not exempted from assassination.”

As she left the building flanked by police officers, Ms. Ressa addressed reporters, telling them, “I will do the right thing.”