In a statement last week, Dela Rosa said: “There was an alarming report that communist terrorists are recruiting minors as their fighters as young as 10 years old.”

On Wednesday, several tearful parents went to the Senate to recount that their children had been recruited by communist groups and had then gone missing.

Talking in Tagalog to a radio station on Saturday, Dela Rosa advised: “We parents, let’s observe our children for any changes in their habits, or their outlook on their lives.”

He said these could be indications the children had been brainwashed by communists.

“You should take steps as parents to avoid losing your children [to leftist groups] altogether,” the senator said.

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On Sunday Dela Rosa added that police and the military should be allowed to enter state-run universities to “indoctrinate” students and counter recruitment efforts by leftist organisations.

On August 1, the Philippine National Police filed charges of kidnapping, child abuse and crimes against humanity against youth leftist group Anakbayan over the “disappearance” of a 17-year-old student it had recruited.

Anakbayan replied in a statement that members join voluntarily. “Every member is free to join and leave at any time,” it said.

Calling Dela Rosa “an utter fool”, Anakbayan spokesperson Alex Danday said “children grow into mature adults, unlike him”.

He added: “Changing means we have learned to ask questions, to tell right from wrong, and make our own decisions – something Dela Rosa has never learned in his 57 years.”

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She had gone home in May but her parents refused to let her leave the house and instead brought her to a military camp to make her “normal”. That was when she left her parents.

Renato Reyes, secretary general of the leftist party-list group Bayan Muna, which occupies one seat in Congress, tweeted that people joined leftist groups in various ways.

“Not all are walk-ins. Some needed a little nudge. Two years into being an activist at the University of the Philippines, I recruited my teacher.

“I first had to get my grades before I asked,” he said.

Duterte’s national security adviser, ex-military chief Hermogenes Esperon, also joined leftist activities as a student. Esperon told the South China Morning Post in a 2007 interview it was due to peer pressure and fervour.

“It was easy for me to empathise with the masses because I was a farmer.”

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When Duterte became president, he seemed to have come to an accommodation with the communists. The party’s founder, Jose Maria Sison, had been Duterte’s political science teacher in college. Duterte himself disclosed during his 2016 election campaign that he had previously joined Sison’s left-wing youth group Kabataang Makabayan (Nationalist Youth).

The president appointed several communists to cabinet positions, but in 2017, he declared both the Communist Party and NPA terrorist organisations, saying they wanted to overthrow his administration. He vowed to destroy them.

The military and police then went after members and officers of legal left-wing organisations such as Anakpawis, which they said secretly recruited for the NPA.

In his statement, Dela Rosa said: “The communist group has been doing this [recruiting minors] for decades but their recruitment efforts are intensified now because their armed group has been depleted due to frequent clashes with government troops.”

The senator, who heads a committee on public order and dangerous drugs and is co-vice-chair of the committee on defence and national security, called for an investigation into the recruitment.