Philippine President Rodrigo Duterte has voided the amnesty of one of his most outspoken critics, ordering police to arrest the Senator who has accused him of profiting from the illegal drugs trade and plotting to assassinate him.

Senator Antonio Trillanes IV said he would remain in the custody of the Senate President until his lawyers could petition against the measure he called “illegal and unconstitutional.”

“What Mr. Duterte wants to happen is to order a warrantless arrest which seems to declare a de facto martial law,” Trillanes said in a statement.

Duterte issued a proclamation Tuesday morning empowering security forces to “employ all lawful means to apprehend him.” At least 40 members of the police and military arrived at the Philippine Senate building in Manila shortly after, according to the local news site GMA News.

Trillanes, a retired naval officer, was granted amnesty by former president Benigno Aquino III over his role leading two mutinies in 2003 and 2007 against Aquino’s predecessor, Gloria Arroyo. Tuesday’s proclamation claimed Trillanes fell short of the minimum requirements, which include an admission of guilt.

Read more: The Mutineer: How Antonio Trillanes Came to Lead the Fight Against Rodrigo Duterte

If detained, Trillanes will become the second opposition lawmaker jailed under Duterte, following the arrest nearly 18 months ago of Leila De Lima shortly after she launched a probe into alleged extrajudicial killings carried out as part of his administration’s bloody war on drugs.

The following month, the government’s top lawyer threatened to impeach Vice President Leni Robredo and accused her of treason after she criticized his drug war. This May, another prominent critic, Chief Justice Maria Lourdes Sereno, was ousted from the Supreme Court.

Author and analyst Richard Heydarian tells TIME that as the president’s top critics increasingly end up in jail or out of power, it will become “increasingly difficult for the government to argue that what’s going on isn’t a crackdown on the opposition.”

“The move against Trillanes could also politically backfire,” he adds, “transforming him into a ‘hero,’ not only for the political opposition but a larger section of the society.”

In a statement, minority block senator Francis Pangilinan called the move “clear persecution” intended to “silence Sen. Trillanes, who in the past has exposed to the public possible wrongdoings of the President.”

Vice President Robredo said the order proves that the Duterte administration will do anything to silence his critics, and called it a diversionary tactic meant to divert public attention away from the country’s economic problems, Philstar reports.

Duterte is currently on a diplomatic mission to Israel. His proclamation on Trillanes amnesty comes two months after the senator said the PNP had withdrawn his security detail.

Read more: Exclusive: Inside the Cell of Leila de Lima, Duterte’s ‘First Political Prisoner’

In an interview with TIME last year, Trillanes called Duterte’s bloody drug war a “sham” designed to “deflect suspicion” from his own family, who Trillanes alleges have ties with the illegal drug trade coming through the southern city of Davao.

The enmity goes both ways. Duterte has said of Trillanes: “I destroy him or he will destroy me.”

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Write to Joseph Hincks at joseph.hincks@time.com.