Khurram Pervez was arrested late on Thursday after returning to his home in Srinagar, which has been roiled for months by violent protests over the killing of a young separatist by Indian soldiers.
Police superintendent Faisal Qayoom confirmed his arrest but did not say what the charges were.
“We are looking into it. For the moment we’ve taken him into custody,” he said.
Pervez’s wife Samina said police had come to the family home late on Thursday to arrest him. He can be held for up to six months without charge under India’s Public Safety Act.
More than 100 people have been killed in Indian-held Kashmir since Burhan Wani’s death on July 8, in one of the deadliest bouts of violence since a full-blown armed rebellion was at its peak in the 1990s.
Most have died in clashes between protesters and police and paramilitaries who have fired tear gas and pellet guns at demonstrators.
Authorities this week banned prayers to mark the Muslim festival of Eid-ul-Azha at the main mosque in Srinagar.
Internet and mobile networks have also been cut off in a bid to prevent a repeat of the protests.
Pervez, a coordinator of the Jammu Kashmir Coalition of Civil Society (JKCCS), was scheduled to brief a UN Human Rights Council session on the situation, but immigration officials blocked him from boarding his flight at Delhi’s main airport.
Both India and neighbouring Pakistan lay claim to the whole of the Himalayan territory, which has been divided between the two since they separated seven decades ago.
The two countries, which are both now nuclear powers, have twice gone to war over the territory and accuse each other of stoking violence.