Imran Khan at PM House Islamabad.
His reaction came after the Indian government earlier in the day decided to revoke Article 370 that gives special status to Jammu and Kashmir and split the state into two Union Territories: Jammu and Kashmir with an Assembly and Ladakh without one.
The cricketer-turned-politician termed changing the status of Jammu and Kashmir as a "violation of UN resolutions."
"India's move will further deteriorate relations between nuclear-capable neighbours," he said.
On Sunday, Imran said that Pakistan will respond to any "misadventure or aggression" by Indian forces against it.
Chairing the National Security Committee meeting in Islamabad, Imran alleged that India has used "cluster bombs" on civilians along the Line of Control and called on the UN Security Council to take note of the "international threat to peace and security."
He said that Pakistan "will always stand with Kashmiris and will not be deterred from its just stance based on United Nations Security Council's resolutions and aspirations of Kashmiri people."
Imran's move comes in the wake of US President Donald Trump renewing his offer to mediate in the Kashmir issue, which India has firmly rejected.
The official Twitter handle of President Arif Alvi said: "India's attempt to further change status of Indian-occupied Jammu and Kashmir is against the resolutions of UNSC and against wishes of the Kashmiri people."
"Pakistan supports and insists on a peaceful resolution based on wishes of Kashmiri people and stands with them in their hour of need," he said in another tweet.
Earlier, a statement from Pakistan's Foreign Office said: "The Indian Occupied Jammu & Kashmir is an internationally recognised disputed territory."
It said that "no unilateral step" by the Indian government can "change the disputed status" according to the United Nations Security Council (UNSC) resolutions.
"Nor will this ever be acceptable to the people of Jammu & Kashmir and Pakistan."
It said that as party to this international dispute, "Pakistan will exercise all possible options to counter the illegal steps."
"Pakistan reaffirms its abiding commitment to the Kashmir cause and its political, diplomatic and moral support to the people of Occupied Jammu and Kashmir for realisation of their inalienable right to self-determination," it said.
Foreign Minister Shah Mahmood Qureshi said that Islamabad will highlight its stand with the US and international community over the issue.
"Pakistan condemns and rejects announcements made today by the Indian government regarding Indian-occupied Jammu and Kashmir," he tweeted.
"We intend to firmly highlight our stance in our meetings with the US delegation visiting Pakistan and with the international community at large," he said.
Pakistan Muslim League-Nawaz (PML-N) President and opposition leader Shehbaz Sharif also condemned India's move, declaring it "unacceptable" and an "act of treason" against the UN.
Calling for an emergency meeting of the parliamentary leaders, Sharif said: "This is a matter of Pakistan's national interest and the entire nation is united on this front."
Condemning India's move on Kashmir, Pakistan Peoples Party (PPP) Chairperson Bilawal Bhutto Zardari tweeted: "Atrocities in Indian-occupied Kashmir (IoK) unabated. Extremist Indian govt's intentions clear. President must immediately summon Joint session of Parliament in wake of Indian aggression in IoK."
Indo-Asian News Service
The premier's message came as the curfew and communications blackout imposed by New Delhi in Kashmir entered its 12th day. Aug.15 is also India's Independence Day, which is being observed as a black day across Pakistan to protest the brutalities in Kashmir.
India's decision to strip Kashmir of the special autonomy it has enjoyed for seven decades has also prompted condemnation on Pakistani Twitter, where at one point the top six trending hashtags all related to the move.
As per a statement issued by the Prime Minister Office, the decision has been "taken in view of the regional security environment.” Bajwa's tenure was due to end in November but analysts have long predicted the extension.
Qureshi paid tribute to those soldiers who were martyred while defending the region and those who were protecting the Line of Control (LoC) even on the occasion of Eid. He also assured Kashmiri people of Pakistan's support.
In renewed airstrikes on Thursday, Israel destroyed a six-storey residential building in the heart of Gaza City. One man was killed by an Israeli missile fired east of Khan Younis in southern Gaza, medics said.
President Dr Arif Alvi and Prime Minister Imran Khan called on citizens to strictly adhere to anti-virus protocols while celebrating Eid Al Fitr, stating that the measures were necessary to avoid a massive outbreak.
Home Minister Mohammed Mahmood Ali was informed about the decision and he told the Muslim leaders that he would speak to the Director General of Police.