MANILA - The Philippine government on Monday detained a 71-year-old Australian nun for "illegal political activities" after she took part in a human rights fact-finding mission in the troubled south and she was expected to be deported, a leader of a leftwing group said.

Her detention came a day after Giacomo Filibeck, a Socialist Party official from the European Union who had criticized Duterte's brutal anti-drugs crackdown, was deported.

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Sister Patricia Fox was taken from her house and brought to the immigration bureau in Manila, said Renato Reyes, secretary-general of the leftist Bayan movement.

The immigration department informed her about deportation proceedings against the nun, she said.

"We condemn her unjust detention and the deportation proceedings initiated against her," Reyes said. "She is no criminal or undesirable alien. She has long been in the Philippines helping the poorest of the poor."

Fox is the superior of the Notre Dame de Sion in the Philippines, a congregation of Catholic nuns. She also told her lawyer, Sol Taule, the government would deport her, Reyes told Reuters.

Fox had been involved with human rights missions in Mindanao, looking into the plight of indigenous people and farmers, leftwing lawmakers said, almost a year after President Rodrigo Duterte imposed martial law on the southern island of Mindanao.

The immigration bureau confirmed the Fox's arrest but declined to issue any statement until after the investigation is complete. Fox was unavailable for comment.

Leftist lawmakers have vowed to hold a congressional inquiry into the deportation of foreign human rights advocates.

"The immigration department is barking at the wrong tree on this one. Helping the poor is not a crime and joining peace activities to advocate peasant welfare and human rights is not against the law," the lawmakers said in a statement, calling for her immediate release.