India's government has revoked part of the constitution that gives Indian-administered Kashmir special status, prompting fears of unrest.
Article 370 is sensitive because it guarantees significant autonomy for the Muslim-majority state.
The measure was accompanied by a telecoms and media blackout which began on Sunday evening.
There is a long-running insurgency on the Indian side. India and Pakistan fought several conflicts over Kashmir.
For many Kashmiris, Article 370 was the main justification for being a part of India and by revoking it, the BJP has irrevocably changed Delhi's relationship with the region, the BBC's Geeta Pandey reports from Delhi.
Meanwhile India's parliament is expected to pass a bill splitting Indian-administered Kashmir into two territories governed directly by Delhi.
Pakistan condemned India's decision to revoke the special status of its part of Kashmir as illegal, saying it would "exercise all possible options" to counter it.
"India is playing a dangerous game which will have serious consequences for regional peace and stability," said Foreign Minister Shah Mehmood Qureshi.
But an Indian government source said there was no external implication as the Line of Control, the de facto border, and boundaries of Kashmir had not been altered.
Why are there tensions over Kashmir?
During the partition of the Indian subcontinent in 1947, some expected Jammu and Kashmir, like other Muslim-majority regions, to go to Pakistan.
But the ruler of the princely state, who had initially wanted Jammu and Kashmir to become independent, joined India in return for help against an invasion of tribesmen from Pakistan.
War broke out between India and Pakistan, and Kashmir effectively became partitioned.
The region, which remains one of the most militarised zones in the world, has been a flashpoint between India and Pakistan for more than six decades.
Atmosphere of fear
By Aamir Peerzada, BBC News, Srinagar
By the time we woke up this morning, the internet was gone and we now have no mobile connectivity.
If people step out of their homes, they see paramilitary forces on every street. Almost every major road is shut - we are hearing that more troops are being deployed.
No-one knows what is happening in other parts of the state - we can't talk to anyone else.
People are concerned - they don't know what is happening, they don't know what is going to happen.
It's an atmosphere of fear. People are scared to come out, they have stockpiled food for months.
Kashmiris have always been willing to defend the state's special status. It looks like a long road ahead, and no-one knows what's next.
What is Article 370?
In 1949, a special provision was added to India's constitution providing autonomy to Jammu and Kashmir.
Article 370 allows the state to have its own constitution, a separate flag and independence over all matters except foreign affairs, defence and communications.
Another provision later added under Article 370 - 35A - gives special privileges to permanent residents, including state government jobs and the exclusive right to own property in the state.
It is seen as protecting the state's distinct demographic character as the only Muslim-majority state in India.
So why is India's move controversial?
The move by the Hindu nationalist BJP government prompted outrage in parliament, and some legal experts have called it an attack on the constitution.
Critics fear the move is designed to change the demographic make-up of India-administered Kashmir - by giving people from the rest of the country to right to acquire property and settle there permanently.
"They just want to occupy our land and want to make this Muslim-majority state like any other state and reduce us to a minority and disempower us totally."
Why is the government doing this?
The ruling BJP made revoking Article 370 part of the party's 2019 election manifesto - and it won a landslide victory earlier this year.
It has argued that Article 370 has prevented the region's development and its integration with India.
An Indian government source told journalists the region's special status had discouraged outside investment and affected its economy, while terrorism and smuggling were rife.
"A set of anachronistic provisions were not allowing the progress of Kashmir," the source said. "The huge sum of money and resources which were going into the state were not being optimised."
How did the government make the change?
India's government announced a presidential order revoking all of Article 370 apart from one clause which says that the state is an integral part of India.
The order was met by massive protests from the opposition - but has now been signed into law by President Ram Nath Kovind.
Prior to the announcement, a telecoms and media blackout began on Sunday evening in the region. The government explained the move as being aimed at pre-empting any violence that the announcement might trigger.
Officials said the restrictions would not be in place for long.
Parliament is also expected to approve a measure dividing the state into two regions ruled by the central government.
One region will combine Muslim-majority Kashmir and Hindu-majority Jammu. The other is Buddhist-majority Ladakh, which is culturally and historically close to Tibet.
What has been happening in Kashmir?
Indian-administered Kashmir is in a state of lockdown.
Curfew-like conditions have been imposed, and orders preventing the assembly of more than four people have been introduced.
Tens of thousands of Indian troops were deployed to the region ahead of Monday's announcement and tourists were told to leave under warnings of a terror threat.
In the hours before Monday's announcement, two of the state's former chief ministers - Omar Abdullah and Mehbooba Mufti - were placed under house arrest.