UAE President Sheikh Khalifa bin Zayed Al-Nahyan died Friday aged 73 after a years-long battle with illness, triggering a period of mourning and a handover of power in the oil-rich Gulf state.

Sheikh Khalifa, who was in office during a time of rocketing fortunes for the United Arab Emirates but was rarely seen in public, was laid to rest in Abu Dhabi's Al Bateen Cemetery just hours after his death was announced, in accordance with Muslim tradition.

UAE President Sheikh Khalifa bin Zayed seen in a file picture from May 2022 UAE President Sheikh Khalifa bin Zayed seen in a file picture from May 2022 Photo: AFP / ADAM JAN

Sheikh Khalifa's funeral prayers were conducted by his expected successor and half-brother Mohamed bin Zayed, who has long been viewed as the country's de facto ruler.

"The people of the Emirates are united in grief as we mourn the loss of our leader and President, Khalifa bin Zayed," tweeted Mohamed bin Zayed, often known as "MBZ".

UAE President Sheikh Khalifa bin Zayed Al-Nahyan (R) is seen at the 2009 wedding ceremony of Sheikh Nasser bin Hamad Al-Khalifa (C), son of Bahrain's King Sheikh Hamad, and Dubai's ruler Sheikh Mohammed bin Rashid Al-Maktoum (L) UAE President Sheikh Khalifa bin Zayed Al-Nahyan (R) is seen at the 2009 wedding ceremony of Sheikh Nasser bin Hamad Al-Khalifa (C), son of Bahrain's King Sheikh Hamad, and Dubai's ruler Sheikh Mohammed bin Rashid Al-Maktoum (L) Photo: BNA (Bahrain News Agency) via AFP

The most visible testament to Sheikh Khalifa is the world's tallest building, Dubai's Burj Khalifa, which was renamed after he bailed out the debt-hit emirate when the global financial crisis struck in 2009.

Flags flew at half-mast above the interior ministry in Abu Dhabi and other landmarks at the start of 40 days of official mourning, with work suspended in the public and private sectors until Tuesday.

Factfile on UAE's President Sheikh Khalifa bin Zayed Al-Nahyan, who died Friday Factfile on UAE's President Sheikh Khalifa bin Zayed Al-Nahyan, who died Friday Photo: AFP / Sylvie HUSSON

Emirati men in traditional dress attended sombre funeral prayers at Abu Dhabi's enormous, white-marble Sheikh Zayed Grand Mosque, where the imam's voice brimmed with emotion over the loudspeaker.

As tributes poured in from abroad, US President Joe Biden called Sheikh Khalifa "a true partner and friend of the United States". Neighbouring Saudi Arabia suspended sports and entertainment events and Oman, Lebanon and Kuwait also ordered three days of mourning.

In a demonstration of the UAE's diverse allegiances, Russian President Vladimir Putin also paid tribute, as did Iran's foreign minister and Israeli Prime Minister Naftali Bennett.

A local channel displays the portrait of late UAE President Sheikh Khalifa bin Zayed Al-Nahyan, during state mourning in Abu Dhabi A local channel displays the portrait of late UAE President Sheikh Khalifa bin Zayed Al-Nahyan, during state mourning in Abu Dhabi Photo: AFP / Karim SAHIB

Sheikh Khalifa took over as the UAE's second president in November 2004, succeeding his father as the 16th ruler of Abu Dhabi, the richest of the federation's seven emirates.

He has made few public appearances since 2014, when he had surgery following a stroke, although he has continued to issue rulings.

Emirati men in traditional dress attended sombre funeral prayers at Abu Dhabi's enormous, white-marble Sheikh Zayed Grand Mosque, where the imam's voice brimmed with emotion over the loudspeaker Emirati men in traditional dress attended sombre funeral prayers at Abu Dhabi's enormous, white-marble Sheikh Zayed Grand Mosque, where the imam's voice brimmed with emotion over the loudspeaker Photo: AFP / Karim SAHIB

The cause of death was not immediately released.

The United Arab Emirates flag flies at half-mast outside the Sheikh Zayed Grand Mosque in Abu Dhabi The United Arab Emirates flag flies at half-mast outside the Sheikh Zayed Grand Mosque in Abu Dhabi Photo: AFP / Karim SAHIB

The UAE, a former British protectorate that was founded in 1971, has gone from desert outpost to booming state in its short history, fuelled by its oil wealth and Dubai's rise as a trading and financial centre.

The Arab world's second-biggest economy behind Saudi Arabia is now wielding growing political influence, filling a space ceded by traditional powers such as Egypt, Iraq and Syria.

The country of about 10 million also joined military campaigns in Libya and Yemen and broke ranks with much of the Arab world to establish ties with Israel in 2020.

As the changes unfolded, the bearded Sheikh Khalifa had cut a frail figure on his occasional public appearances, while his half-brother hosted world leaders and led diplomatic forays abroad.

Sheikh Khalifa, who had no formal higher education, came to the rescue of Dubai when it was hit by the global financial crisis, extending a multi-billion-dollar lifeline to the emirate.

Dubai's ruler, UAE Vice President Sheikh Mohammed bin Rashid Al-Maktoum, said the country mourned "with hearts filled with sadness".

The emir of Qatar -- which was blockaded by the UAE, Saudi Arabia and other states in a three-and-a-half-year crisis until January 2021 -- offered his condolences, as did British Prime Minister Boris Johnson and French President Emmanuel Macron.

In Saudi Arabia, the UAE's powerful neighbour and closest ally, King Salman ordered funeral prayers at Mecca's Great Mosque and the Prophet's Mosque in Medina.

Saudi's de facto ruler Crown Prince Mohammed bin Salman, who was regarded as a protege of Mohamed bin Zayed, phoned him with his condolences.

The leaders of the seven emirates elect the UAE's president for five-year, renewable terms, according to the country's constitution.