Veteran Kashmiri separatist leader and former chief of the All Parties Hurriyat Conference (APHC) Syed Ali Shah Geelani passed away on Wednesday, APHC leader Abdul Hameed Lone said.
In a statement, Lone said Geelani died at his home at 10pm.
He was 92 years old, according to the Kashmir Media Service.
Meanwhile, Indian authorities imposed a security clampdown in occupied Kashmir after Geelani's death, residents said.
Troops put up barbed wire rolls and barricades on roads leading to Geelani's house in the main city of Srinagar after the family announced the death.
Hundreds of security forces were immediately deployed and media reports said a curfew would be imposed and internet services cut.
Announcements were made from loudspeakers of the main mosque near Geelani's residence asking people to march towards the house. But scores of armoured vehicles and trucks patrolled main roads in the area. Police appealed for people not to go out on the street.
Uncompromising campaigner against Indian rule
Geelani, an uncompromising campaigner against Indian rule in Kashmir, had been under house arrest for the past 11 years. He had been ill for several months.
Geelani had been a thorn in India's side since the early 1960s when he began campaigning for the territory's merger with Pakistan. He also pursued his separatist calls as a member of the Kashmir assembly.
The veteran politician was jailed for nearly 10 years after 1962 and often restricted to his home after that.
Since his youth, Geelani had been a member of Jamaat-i-Islami, which was banned by the Hindu nationalist government in 2019.
Last year, Geelani had resigned from his post as APHC chief, of which he had been a member since its formation in 1993 as a political arm of the anti-India movement. He was elected its chairman for life in 2003. Hurriyat Conference (Geelani) has more than 24 constituent parties, some of which have only a handful of members.
Last month, the APHC had said that the 11-years-long house arrest had taken a heavy toll on Geelani's health and his condition continued to deteriorate.
In a press release, a copy of which was shared by the Kashmir Media Service, the APHC said that along with general physical weakness that the veteran leader had been experiencing for some time, his chest infection was also "not satisfactorily" responding to treatment.
Pakistan to observe day of mourning
Prime Minister Imran Khan said he was "deeply saddened" to learn about Geelani's death.
Recalling his courageous struggle, the premier said Geelani had struggled all his life for his people and their right to self-determination.
"He suffered incarceration and torture by the occupying Indian state but remained resolute.
"The Pakistan flag will fly at half-mast and we will observe a day of official mourning," the premier announced.
President Dr Arif Alvi had last year conferred Nishan-i-Pakistan, the country's highest civil award, on Geelani in recognition of his decades-old struggle for Kashmiris' right to self-determination.
Terming Geelani the "torchbearer of the Kashmir freedom movement", Foreign Minister Shah Mahmood Qureshi said Pakistan mourned the veteran leader's death.
"[Geelani] fought for the rights of Kashmiris till the very end, under house arrest of Indian occupation. May he rest in peace and may his dream of freedom come true," he tweeted.
Former chief minister of Indian-occupied Jammu and Kashmir, Mehbooba Mufti, also expressed sadness at Geelani's passing.
"We may not have agreed on most things but I respect him for his steadfastness & standing by his beliefs," she said.
She prayed for Geelani and conveyed her condolences to Geelani's family and well wishers.