Pakistan's political and military leaders have warned India that they will challenge the Hindu-majority nation's move to strip its portion of disputed Kashmir of its special status at "every forum".
India on Monday dropped a constitutional provision for the state of Jammu and Kashmir, which has long been a flashpoint in ties with neighbouring Pakistan, to make its own laws.
Nuclear arch-rivals India and Pakistan both claim Muslim-majority Kashmir in full but rule it in part. They have fought two wars over the territory and came close to a third this year after a car bomb set off by a Pakistan-based militant group killed dozens of Indian paramilitary police.
Pakistan Prime Minister Imran Khan said on Tuesday the country was considering an approach to the United Nations Security Council over the move.
"We will fight it at every forum. We're thinking how we can take it to International Court (of Justice)... to the United Nations Security Council," Mr Khan said in an address to Pakistan's parliament.
"If the world does not act today... (if) the developed world does not uphold its own laws, then things will go to a place that we will not be responsible for," he added.
Pakistan's military "firmly stands" by Kashmiris, the powerful army chief said before Mr Khan's comments.
The army's top commanders met in the garrison city of Rawalpindi to discuss the move by India, which is also set to exacerbate the long-running bloody rebellion in Kashmir.
"Pakistan Army firmly stands by the Kashmiris in their just struggle to the very end. We are prepared and shall go to any extent to fulfil our obligations in this regard," General Qamar Javed Bajwa said, in a tweet sent by a military spokesman after the meeting.
Spokesman Major General Asif Ghafoor said commanders "fully supported" the civilian government's rejection of India's move.
"Pakistan never recognised the sham Indian efforts to legalise its occupation" of the disputed mountainous region, he added.
In India, there was celebration over what ministers called a "historic" decision to bring Kashmir under its direct control.
Three high-profile political leaders in Kashmir were detained for being a threat to peace hours after after Home Affairs Minister Amit Shah announced the decision.
Mr Shah, a powerful aide of Prime Minister Narendra Modi, compared the move to other "historic moments" in the nation, adding that the law should be "written with golden words in Indian history".
"The great parliament which has expressed its views on unity and integrity of India so many times today is once again going to deliberate on a bill that will go on to further integrate Jammu and Kashmir to India for many generations ahead," he told parliament Tuesday.
His declaration came as reports emerged on Tuesday of sporadic protests and at least six people admitted to a hospital in the main city Srinagar with gunshot wounds and other injuries.
Indian-administered Kashmir has been in the grip of a bloody rebellion against Indian rule since 1989, and analysts have warned the scrapping of its autonomous status could trigger fresh protests.
The region has been divided between India and Pakistan since their independence in 1947.
In India, criticism mounted from opposition politicians on Tuesday, with Rahul Gandhi, until recently the head of the main opposition Congress party, saying the decision was an "abuse of executive power" that had "grave implications for our national security".
"National integration isn't furthered by unilaterally tearing apart J&K, imprisoning elected representatives and violating our Constitution. This nation is made by its people, not plots of land," he wrote on Twitter.
Pakistan was braced for protests on Tuesday, with demonstrations set to kick off in the early afternoon with protests planned in Muzaffarabad, the largest city in Pakistani-held Kashmir, alongside rallies in Lahore, Karachi, and the capital Islamabad.
