Khurram Parvez was arrested Monday night by India’s National Investigation Agency (NIA), an Indian official briefed on the situation told Reuters.
His residence and office were searched and a mobile phone, laptop and books were confiscated, he added.
An NIA spokesperson confirmed Parvez’s arrest Tuesday.
He is detained under the Illegal Activities (Prevention) Act, which allows detention for up to six months without trial.
His attorney, Parvez Imroz, could not immediately be reached for comment.
Mary Lawlor, the UN Special Rapporteur on Human Rights Defenders, called Parvez’s arrest “disturbing.”
“He is not a terrorist, he is a defender of human rights,” he said in a tweet.
Parvez, one of Kashmir’s best-known activists, is director of the Jammu Kashmir Civil Society Coalition, a group of rights organizations working in the region.
He was arrested and detained on similar charges in 2016, after he was prevented from boarding a flight to attend a UN human rights forum in Geneva. He was finally released without having been convicted of any crime.
The Muslim-majority Kashmir region has been the source of decades of tension between nuclear-armed neighbors India and Pakistan.
Both countries claim the Himalayan territory in its entirety, but rule it in part, and have fought two wars with each other there.
India has long faced allegations of rights abuses in its part of the territory, charges that New Delhi denies.
It strictly controls access to Kashmir for foreign observers, including the UN.