MANILA — President Rodrigo Duterte has ordered the arrest of one of his most prominent critics in the Philippine Senate, declaring that an amnesty granted to the senator years ago over his role in two military uprisings was invalid.

The senator, Antonio Trillanes, a former naval officer, called Mr. Duterte’s order the act of a dictator and said he would not resist arrest. If he is detained, he will be the second well-known Duterte critic in the Senate to be put behind bars.

“If the ordinary folk are being killed, he is now jailing his critics,” Mr. Trillanes said Tuesday, referring to Mr. Duterte’s bloody crackdown on narcotics. “That is the situation in the Philippines.”

The president’s order, which was signed last week and made public on Tuesday, declares that the amnesty granted to Mr. Trillanes in 2010 by Mr. Duterte’s predecessor, Benigno S. Aquino III, was “void ab initio,” or invalid from the start.