(UPDATED) Human rights activist Natividad "Naty" Castro has been detained at the Bayugan City police station in Agusan del Sur, authorities said on Saturday.

According to Police Maj. Dorothy Tumulak, spokeswoman of Police Regional Office 13, Castro, a human rights activist who is a former secretary-general of Karapatan-Caraga, was brought to Butuan City on Friday night.

A Bayugan court issued the arrest warrant on Castro.

Various rights and lawyers groups earlier on Saturday expressed concern over the whereabouts of Castro, who was arrested on Friday by the PNP Intelligence Group in San Juan City over charges of kidnapping and serious illegal detention in the CARAGA Region.

• Rights groups, lawyers ‘gravely concerned’ over Naty Castro’s whereabouts

Castro, 53, is being held without bail and will be tried on charges of kidnapping, for which a lower court had ordered her arrest in 2020, according to a police statement issued Friday night.

But her former employer, human rights monitor Karapatan, said Castro was facing "trumped up" charges after she investigated alleged human rights violations in the volatile Mindanao region while also setting up community health centers there.

St. Scholastica's College Manila said Castro's arrest was "ridiculous."

Castro graduated valedictorian in 1986 in St. Scholastica's College Manila and was named one of its most outstanding alumnae.

"It is unjust that one who has chosen to live in places that are not reached by the services that every human being is entitled to receive; one who has committed her life to give life to others is now deprived of her right to life, a life that she has lived witnessing to Christ’s love and compassion," the school said in a statement.

She was arrested at her home on Friday, and was allegedly accused of being part of the Communist Party of the Philippines' central committee.

"Karapatan denounces the arrest of human rights and health worker Dr. Naty Castro as yet another form of attack against human rights defenders," the group said, referring to Castro by her nickname.

Senatorial candidate Dr. Minguita Padilla also expressed concern over the arrest of Castro.

Padilla urged authorities to respect her rights, especially that Padilla is among frontliners sacrificing their safety to serve the public.

"I pray that our law enforcers respect her Constitutional rights, and that she is given fair and proper treatment," Padilla said in a statement.

"Let us remember that Dr. Natividad is part of the frontline healthcare workers, who have risked their lives to serve our country, especially during this pandemic."

Critics say allegations of communism -- known locally as "red-tagging" -- have been used to discredit and detain activists, journalists, lawyers and dissidents.

While the practice is not new in the Philippines, it has intensified under President Rodrigo Duterte, according to rights groups.

At the start of his term in 2016, Duterte, a self-described socialist, had sought a peace deal with the rebels.

But after talks collapsed in 2017, he branded the Communist Party and its armed wing "terrorist organizations" and ordered soldiers to shoot female insurgents in the genitals.

PNP: No red tagging in Castro case

PNP Public Information chief Brig. Gen. Roderick Alba said Castro was arrested not because of her alleged membership with the communist movement.

"It must be understood that the accused is wanted by the law to stand trial in Criminal Case No. 6527," Alba said in a statement. "Her appearance in court will allow her to further enjoy all the guaranteed rights of an accused person."

The Department of Health, for its part, said it trusts law enforcement to uphold the rights of Castro.

"The contributions of our health workers, especially those who have opted to work with the underserved are immeasurable," the DOH said in a statement.

"All our citizens, health workers included, enjoy the constitutional guarantees of due process and presumption of innocence until proven guilty. We trust our authorities to uphold these rights."

Castro's brother insisted she was just a health worker who had served indigenous communities.

"My sister is accused of multiple charges of kidnapping and illegal detention, all related to her human rights advocacy. ALL UNTRUE," he wrote.

PNP chief Gen. Dionardo Carlos congratulated police on Castro's arrest, saying in the statement they made "it possible to bring the suspect before the court". — With reports by Lorilly Charmane D. Awitan and Agence France-Presse