- Ceremony in Belgian city of Liege marks anniversary of outbreak of World War I
- Germany invaded neutral Belgium on August 4, 1914, and Britain declared war
- Over the next four years, 16 million people died in the conflict, many on Belgian battlefields
- Prince William said war was now "unthinkable" between nations who had fought
Twelve cannons were fired in Liege, Belgium, on Monday to conclude a ceremony marking the outbreak of World War I 100 years ago.
Britain declared war on Germany after it invaded neutral Belgium on August 4, 1914. The declaration escalated a conflict -- sparked by the assassination of the Archduke of Austria-Hungary -- into a global war.
About 16 million people were killed before the war ended four years later. Belgium was a key WWI battleground.
Dignitaries including King Philippe of Belgium, the French and German Presidents and Britain's Prince William, Duke of Cambridge, addressed those gathered in remembrance Monday.
Prince William said the people of Belgium had been among the war's first victims, and their "resistance was as gallant as their suffering was great."
Belgium's sacrifice and contribution had been pivotal to eventual victory, the prince said.
"Belgium's resistance in 1914 allowed the Allies to regroup and draw up the battle lines which became the infamous trenches. These trenches have left an indelible scar on your land; they speak of the horrors of war but also of your forebears' courage," he said.
Prince William said recent events in Ukraine showed that "instability continues to stalk our continent."
But he said the fact that war was now "unthinkable" between the former World War I enemies represented at the ceremony "bears testimony to the power of reconciliation."
"We were enemies more than once in the last century, and today we are friends and allies. We salute those who died to give us our freedom. We will remember them."
The king of Belgium laid a wreath to commemorate the dead before a bugler sounded "The Last Post."
A young girl representing future remembrance then released a white balloon as a sign of peace and reconciliation before the cannon shots rang out.
Britain is also holding events across the United Kingdom and in the Belgian city of Mons, where the first British soldier died during the conflict.
British Prime Minister David Cameron said in a statement, "A hundred years ago today Britain entered the First World War and we are marking that centenary to honor those who served, to remember those who died, and to ensure that the lessons learnt live with us forever."