PHOTOS: Royals and world leaders unite to commemorate WWI anniversary

Members of the British royal family have joined world leaders in ceremonies in Belgium and Scotland to mark 100 years since the outbreak of World War I.

British Prime Minister David Cameron, Belgian leader Elio De Rupo and French President Francois Hollande were among the 50 or so heads of state who gathered at the ceremony at Saint Symphorien Military Cemetery.

Prince William and his brother Harry addressed the crowds in Belgium.

The Queen attended a ceremony in Abderdeenshire, Scotland, while Prince Charles laid a wreath at a commemoration in Glasgow.

World War I left nearly 10 million dead and 20 million injured, with around 60,000 Australian lives lost.