Khurram Parvez, 39, co-ordinator of the Jammu Kashmir Coalition of Civil Society (JKCCS) has long campaigned against abuses by state forces in the state.
His arrest in September on charges of activities against public order sparked criticism from the United Nations, which said it was a deliberate attempt to obstruct his work.
The Jammu and Kashmir High Court on Friday ordered police to release Parvez due to a lack of evidence.
“It was a very bad experience. And it was good a experience as well...a blessing in disguise. I have learnt so much about the issues faced by prisoners in jails and also about government deficit to address their problems,” Parvez said in Jammu.
“The jail experience will help me in working for the prisoners whose plight is very bad.”
Parvez said some prisoners had psychiatric problems. Others were isolated because their families had no money to visit them.
The JKCCS has published research into the role of security forces in containing a separatist insurgency in Kashmir that first flared a quarter of a century ago.
At least 78 civilians were killed and thousands wounded in more than two months of clashes between protesters and security forces, sparked by the killing of separatist militant Burhan Wani in a joint army and police operation on July 8.
The unrest was the worst in the state for six years, and critics have accused Indian forces of heavy-handedness as they struggle to contain the protests.
Parvez was stopped by authorities at New Delhi airport on September 14.
He had been due to fly to Geneva to attend the UN Human Rights Council.
He was detained on September 16, released four days later and then placed in preventive detention under a law allowing people to held for up to two years without judicial intervention.
The J&K High Court said Parvez’s detention was illegal, and that law enforcement authorities had abused their powers by ordering his arrest.
Police officials declined to comment on the activist’s release from the Joint Interrogation Centre in Jammu, a facility used to detain and question people suspected of militancy.
The JKCCS said Parvez’s detention highlighted the plight of many Kashmiris who are falsely accused under archaic laws and their fight for justice.
“The struggle for the release of Khurram Parvez is a part of the larger struggle against unlawful detentions, state impunity and the use of repressive laws,” the Srinagar-based human rights group said in a statement.
Many international rights bodies, including UN Rapporteurs, had campaigned for his release.
Parvez said he was happy to be back with his family.
