Thousands of people are on the streets of Saudi Arabia mourning King Abdullah bin Abdulaziz who died overnight.

Video images showed Abdullah's body being carried at his funeral in the capital, Riyadh.

He will be buried this afternoon following the funeral at the Imam Turki bin Abdullah mosque.

Muslim dignitaries from around the world arrived in Riyadh for the funeral, under heavy guard.

State television aired images of the prayer ahead of his burial. The footage showed Abdullah's body shrouded in a simple beige cloth in line with Islamic tradition.

Abdullah, who took power in 2005 following the death of King Fahd, died in hospital at 1.00am local time. He is believed to be aged 90 or 91.

A television statement announced his death and said his half-brother Crown Prince Salman had succeeded to the throne.

"With great sadness and mourning His Royal Highness Crown Prince Salman bin Abdulaziz al-Saud, and all members of the family and the nation, express condolences for the Custodian of the Two Holy Mosques King Abdullah bin Abdulaziz al-Saud," the statement said.

The late monarch's half-brother Moqren has been confirmed as the new Saudi Crown Prince.

The public will be invited to pledge allegiance to the new monarch and Crown Prince at the royal palace later today.

The British Department of Culture, Media and Sport announced that flags would be flown at half mast at Westminster following the death.

Abdullah had run the country as de factor leader since the mid-1990s after his predecessor suffered a debilitating stroke.

He was admitted to hospital on 31 December suffering pneumonia and the royal court announced that he was breathing with the aid of a tube.

Sky's Middle East correspondent Sherine Tadros said the death comes after weeks of speculation and rumour about the monarch's health.

"He's been ill for several weeks. He was admitted to hospital in December with a liver infection and there have been rumours over the past few weeks that he had died," she said.

His successor King Salman, 79, became Crown Prince in June 2012 after the death of Prince Nayef bin Abdulaziz.

He has represented Abdullah at recent public events due to the late king's ailing health.

In a televised speech after the death was announced he vowed to continue the policies of his predecessors.

"We will continue adhering to the correct policies which Saudi Arabia has followed since its establishment," King Salman said.

"The Arab and the Islamic nations are in dire need of solidarity and cohesion," he added.

World leaders have paid tribute to Abdullah, who encouraged reforms to the oil-rich kingdom including greater freedom for women and economic deregulation.

During his reign, Abdullah worked to counter the influence of Saudi Arabia's chief rival Iran.

On hearing of his death, Iran offered condolences and said Foreign Minister Mohammad Javad Zarif would travel to Riyadh for an "official ceremony".

Abdullah and his fellow Sunni Arab monarchs also staunchly opposed the wave of pro-democracy uprisings across the Middle East since 2011.

Saudi Arabia has been among several Gulf countries taking part in a US-led air campaign against Islamic State.

His nation also unflinchingly led Gulf countries in ignoring calls by other producers to cut oil output to curb the sliding price of crude.

He also launched projects to build new economic cities, universities and high-speed railways.

US President Barack Obama expressed his condolences in the wake of Abdullah's death, hailing his "steadfast and passionate belief" in Saudi-American relations.

"As a leader, he was always candid and had the courage of his convictions," Mr Obama said in a statement.