Troops from the The Australian and New Zealand Army Corps (ANZAC) launched their first amphibious assaults on the Gallipoli Peninsula in what is now western Turkey on April 25, 1915, splashing through the waters into a rain of Ottoman gunfire.
Many were killed in the initial assault and the overall Allied campaign ended in failure and evacuation.
But the spirit shown by the Anzac troops has long been seen as critical in forging a national identity in New Zealand and Australia. Both commemorate April 25 as Anzac Day.
The Anzac dawn service as the sun rises above the Aegean is a traditional annual event but gained extra importance in the centenary year, with even greater numbers than normal and an intense security blanket thrown around the area.
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"Like every generation since, we are here on Gallipoli because we believe that the Anzacs represented Australians at our best," said Australian Prime Minister Tony Abbott at the service, which was also attended by New Zealand Premier John Key and the heir to the British throne Prince Charles.
Abbott recalled how the first Anzacs were "tradesmen, clerks, labourers, farmers and professionals" from all levels of society, "ordinary men (who) did extraordinary things".
As the dawn opened, the bugler played the last post and the crowds fell silent for two minutes.
In Australia and New Zealand, record numbers of people also turned out to mark the centenary, with dawn services and parades held across the two countries.
"They loved and were loved in return, were prepared to fight for their beliefs, were, like us, prey to fears and human despair," said Chief of Army David Morrison in an emotive address in Canberra at the Australian War Memorial.
The last Allied soldiers were evacuated in January 1916 with almost no casualties, in stark contrast to the bloody horror of the campaign itself.
"The Gallipoli campaign was a failure, of course; the only really successful part was the evacuation," admitted Abbott.