World leaders and dignitaries, including Britain's Prince William, French President Francois Hollande and German President Joachim Gauck attended the ceremonies noting the August 4, 1914, invasion of neutral Belgium by Germany, which sparked Britain's declaration of war against Germany.
The war ended four years later with an estimated 17 million to 20 million people killed.
King Philippe of Belgium called the anniversary a time for reflection on the responsibility of leaders to preserve peace. Meanwhile, Hollande's address mentioned the impossibility of remaining neutral in light of contemporary events, including the downing of Malaysia Airlines Flight 17 in Ukraine and military conflicts in Iraq, Gaza and Syria.
"During four terrible years, those same Europeans were engulfed by killing and destruction. Among the very first victims were the people of Belgium, whose resistance was gallant as their suffering was great," Prince William said.
The commemoration across Europe unofficially began in July with France's Bastille Day, July 14, declared to honor soldiers from all nations that participated in World War I.