The body of former Cambodian king Norodom Sihanouk was transported to Beijing's airport on Wednesday to be flown back to his homeland for a warm and tearful welcome.

A convoy of about 20 vehicles departed from Beijing Hospital, where Sihanouk died of a heart attack on Monday, including a bus decorated with yellow ribbons and flowers in which a coffin could clearly be seen, an AFP reporter witnessed.

Chinese state television showed the convoy reaching Beijing's airport after traversing normally busy highways that were cleared to allow for its passage

A flight bearing Sihanouk's coffin was due to leave sometime before noon Beijing time (0400 GMT), a Cambodian embassy spokesman told AFP.

Authorities in Cambodia said they expected 100,000 people to line the streets of the capital Phnom Penh to honour Sihanouk, who is a revered figure in his homeland.

Crowds of mourners had already begun to gather in a park outside the royal palace to mark the final homecoming of the man known as the "King-Father".

"I hope to see the royal body, I want to see his face one last time," said Mean Pichavisa, 16, as he sat outside the palace cutting up black ribbons for his school friends to pin to their white shirts in a traditional symbol of mourning.

The youngster, who shaved his head in honour of Sihanouk's passing, said the late monarch's homecoming would be a "historic" moment.

"I will remember this day until I die," he told AFP, as white-robed nuns chanted beside him.

His comments reflect many Cambodians' impressions of the king as a benevolent leader, despite controversies including Sihanouk's harsh crackdowns on critics.

Sihanouk, who steered his country through turbulent decades of war, genocide and finally peace, died at the age of 89.

His corpse will lie in state at the royal palace in Phnom Penh for three months before an elaborate funeral, according to the government.

Sihanouk, who was close to Chinese revolutionary leaders Mao Zedong and Zhou Enlai, fled to China amid the chaos that frequently gripped his country and spent increasing amounts of time in the country as his health failed.

The former king, who had battled illnesses including cancer, diabetes and heart problems in his later years, remained a staunch supporter of China. He saw it as a "second home" and spent much of his time in the country, accompanied by his devoted sixth wife Monique.

His son and current king Norodom Sihamoni and Prime Minister Hun Sen had both arrived in Beijing earlier to collect the body.

Dai Bingguo, China's top official on foreign relations, will accompany the coffin back to Cambodia, according to a statement from the Chinese foreign ministry.

Chinese state media published photos showing flags at Tiananmen Square had been lowered to half-mast in honour of Sihanouk.