In his message on Pakistan's Defence and Martyrs Day, Khan said: "For Pakistan, Kashmir stands as its jugular vein. Altering its status poses challenges to Pakistan's security and integrity.
"I have also urged upon the international community to seriously consider the safety and security of India's nuclear arsenal that is in the control of a racist and Hindu supremacist government; this is an issue that impacts not just the South Asian region but the whole world."
Pakistan observes September 6 as the Defence and Martyrs Day to mark the anniversary of the 1965 war with India.
Tensions between India and Pakistan have heightened after New Delhi revoked the special status for Jammu and Kashmir on August 5 and bifurcated into two union territories.
"Pakistan has launched proactive diplomatic activity in the world capitals as well as in the UN to expose the true face of today's radicalized India," said the cricketer-turned-politician.
"I have informed the world that Pakistan does not want war, but at the same time, Pakistan cannot remain oblivious to the challenges posed to its security and integrity. We are prepared to give the enemy the fullest possible response.
"Failing, the world community will be responsible for the catastrophic aftermath."
Khan said that all of Pakistan was united against India's "illegal, unilateral, reckless and coercive attempt to alter the status of occupied Kashmir" and added that his country would never leave Kashmiris alone, Dawn newspaper reported.
Celebrations to mark the day kicked-off with a 31-gun salute in memory of the martyrs in Islamabad and a 21-gun salute in all the provincial capitals.
Special prayers were offered for the "independence of occupied Kashmir" as well as for the progress and prosperity of Pakistan, Radio Pakistan reported.
In his Defence Day message, President Arif Alvi said: "We will continue raising their just struggle for freedom at all levels and platforms. Indian brutalities and cruelties cannot suppress their spirit of freedom."
Addressing a ceremony at the General Headquarters (GHQ) in Rawalpindi, Chief of Army Staff (COAS) General Qamar Javed Bajwa said that "Pakistan will never leave its Kashmiri brethren and their cause alone and will keep fighting for them".
"I want to make it clear that Kashmir is the unfulfilled agenda of the completion of Pakistan. It will remain so until it is a resolved according to the UN resolutions."
Calling on the international community to play its part, Bajwa said: "Pakistan has fulfilled its responsibilities in the war on terrorism and now the responsibility rests with the rest of the world to play its part."
Foreign Minister Shah Mehmood Qureshi said Pakistan had the capacity to defend the rights of people living in "Indian occupied Kashmir" and referred to them as "brothers" and "sisters".
Paying tribute to the martyrs, Leader of the Opposition in the National Assembly Shehbaz Sharif said that today was a day to express loyalty with the people of Kashmir.
( With inputs from IANS )