"The Ministry of Presidential Affairs has mourned to the UAE people, Arab and Islamic nations and the world the death of President His Highness Sheikh Khalifa bin Zayed Al-Nahyan," the official WAM news agency tweeted.
The ministry announced 40 days of mourning, with flags at half-mast from Friday and work suspended in the public and private sector for the first three days.
Sheikh Khalifa bin Zayed al-Nahyan attends the six-nation Gulf Cooperation Council (GCC) summit in Manama on December 20, 2004
Sheikh Nasir bin Hamad al-Khalifa (C), son of Bahrain's King Sheikh Hamad, standing at his wedding ceremony next to his new father-in-law Sheikh Mohammed Bin Rashid al-Maktoum (L), Vice President and Prime Minister of UAE and Ruler of Dubai, and United Arab Emirates (UAE) President Sheikh Khalifa Bin Zayed al-Nahyan (R) in Dubai on September 28, 2009
Sheikh Khalifa bin Zayed bin Sultan Al Nahyan is seen at Khalifa Port during the official inauguration of the container terminal in Abu Dhabi, December 12, 2012
Sheikh Khalifa is likely to be replaced by his brother, Abu Dhabi Crown Prince Mohammed bin Zayed.
Sheikh Khalifa took over as the UAE's second president in November 2004, succeeding his father as the 16th ruler of Abu Dhabi.
He has rarely been seen in public since 2014, when he had surgery following a stroke, although he has continued to issue rulings.
"The Emirates has lost its virtuous son and leader of the 'stage of empowerment' and the trustee of its blessed journey," the brother of Sheikh Khalifa and Abu Dhabi Crown Prince Mohammed bin Zayed tweeted on Friday.
"His stances, achievements, wisdom, generosity and initiatives are in every corner of the nation.. Khalifa bin Zayed, my brother... may God have mercy on you and grant you access to paradise."
