President Michael D Higgins has attended a ceremony in Liege, Belgium to mark the outbreak of World War I.
The area holds the graves where some of the first army casualties of the war - including a number of Irishmen - are buried.
The event, hosted by the King and Queen of Belgium, was also attended by the presidents of France, Germany and Austria, and by Britain's Prince William.
Over a dozen heads of state from across the globe attended.
German President Joachim Gauck described his country's invasion of neutral Belgium as completely unjustifiable, as were the atrocities committed against civilians and Belgium's cultural heritage.
Prince William said the conflict in Ukraine was a reminder that war still stalked the continent of Europe.
French President Francoise Hollande, striking a more interventionist note, said that Europe could not remain neutral in the face of conflicts in Ukraine and in the Middle East.
This evening President Higgins attended a ceremony at a joint Allied-German cemetery in nearby Mons, hosted by the British government, where the first Irish fatalities occurred in August 1914.
Elsewhere, several hundred people attended a wreath-laying ceremony at the Cenotaph at Belfast City Hall to mark the centenary of the start of WWI.
Guests included members of the loyal orders, representatives from the main unionist political parties and former loyalist paramilitaries.
The service, organised by the Unionist Centenary Committee, was one of a series of events which took place across Northern Ireland today.
A multi-faith service was held at St Anne's Cathedral in Belfast later before a candlelit vigil at the City Hall.