Modi claimed that Article 370, a constitutional provision which had granted special status and powers to the state, had caused "separatism, terrorism, dynastic politics and corruption."
"As a nation, we have taken a historic decision," Modi said. "Due to the past system, the people of Jammu and Kashmir and Ladakh were deprived of many rights, which was a major obstacle in their development. That is now over."
India's parliament voted on Tuesday to reclassify the state of Jammu and Kashmir as a union territory, giving the government in New Delhi greater authority over the country's only Muslim-majority region.
Tens of thousands of additional Indian troops have been deployed into the already heavily militarized region to head off unrest, and several prominent politicians have been placed under house arrest.
The region remains in lockdown amid a communications blackout, with landline connections, internet and mobile coverage all suspended.
On Thursday the United Nations called on both Pakistan and India "to refrain from taking steps that could affect the status of Jammu and Kashmir."
Secretary-General Antonio Guterres said in a statement that he was "concerned over reports of restrictions on the Indian-side of Kashmir, which could exacerbate the human rights situation in the region."
Modi claimed the decision would bring stability to the restive territory. "We will free Jammu and Kashmir of terrorism," he added, referring to the Indian-controlled parts of the disputed region, which is split with Pakistan and China.
"I am confident that the people of Jammu and Kashmir will defeat separatism and move forward with new hopes," Modi said.
Pakistan responded to Delhi's decision by downgrading diplomatic relations and suspending bilateral trade. But Pakistan on Thursday said it is "not looking at military options."
Kashmir is one of the world's most dangerous geopolitical flashpoints and one of the most militarized places on earth. Both India and Pakistan claim Kashmir in its entirety and have gone to war over the region several times.
Pakistani Foreign Minister Shah Mahmood Qureshi told reporters that Islamabad would not engage in the same escalation seen in February, when an attack on Indian paramilitary troops in the Pulwama district of Indian-controlled Kashmir led to the worst cross-border fighting since 1999.
"Pakistan is going to remain vigilant. We are going to place safeguards for our security. We will not be blamed for (a) false flag operation. A 'Pulwama II' as it may. The current Indian (government) is very different from the regimes of the past. The first line of defense is the diplomatic route and that is what Pakistan is engaged in right now. We are not looking at military options," Qureshi said.
Earlier on Thursday, India said it "regrets" Pakistan's response actions and called on Islamabad to review its position "so that normal channels for diplomatic communications are preserved."
India's Foreign Ministry said in a statement: "Recent decisions by the government and parliament of India are driven by a commitment to extend to Jammu and Kashmir opportunities for development that were earlier denied by a temporary provision in the constitution."
"Its impact would also result in the removal of gender and socio-economic discrimination. It is also expected to result in an upswing of economic activity and improvement in the livelihood prospects of all people of Jammu and Kashmir."