According to an immigration official, Faesal was stopped at the Indira Gandhi International Airport as he was about to go abroad. He was sent back to Srinagar on another flight and detained there.
Faesal, who quit the Indian Administrative Service in January to form the Jammu and Kashmir Peoples' Movement (JKPM), has been posting tweets and Facebook content highly critical of the government's move abrogating Article 370 that gave special status to Jammu and Kashmir, and split the state into two Union Territories J&K and Ladakh.
On Monday, he had triggered controversy with his tweet that there would be "no Eid till the last bit of insult is avenged and undone" over what he termed as the "illegal annexation" of Kashmir.
Ahead of Eid, he had tweeted: "There is no Eid. Kashmiris across the world are mourning the illegal annexation of their land."
"There shall be no Eid till everything that has been stolen and snatched since 1947 is returned back."
"No Eid till the last bit of insult is avenged and undone," he posted, in comments that had invited strong criticism on social media.
On Tuesday, he again posted: "Kashmir will need a long, sustained, non-violent political mass movement for restoration of the political rights. Abolition of Article 370 has finished the mainstream. Constitutionalists are gone. So you can either be a stooge or a separatist now. No shades of grey."
In his Facebook post on August 7, he wrote that "Kashmir is experiencing an unprecedented lockdown. It's entire eight million population has been incarcerated like never before".
( With inputs from IANS )