French President Francois Hollande and his German counterpart Joachim Gauck gathered at Hartmannswillerkopf where 30,000 soldiers lost their lives in fierce battles around the mountain peak known as the "man-eater" in France's Alsace region near the border between the two countries.
In a speech lauding as "an example for the world" the friendship between two countries that were once fierce enemies, Hollande remembered conflicts still raging around the world, including the confrontation between Israel and Hamas in Gaza that has claimed over 1,800 lives.
"France and Germany, beyond their suffering and bereavements, had the courage to make up — it was the best way to honour the dead and provide a guarantee of peace to the living," he said.
Their friendship is "an example for the world, a strength and an invitation, wherever peace is threatened, wherever human rights are violated, wherever the principles of international law are flouted.
"All efforts must be made to impose, today more than ever, a ceasefire in Gaza and end the suffering of civilian populations," he added in a speech that also touched on the Ukrainian crisis and the plight of Christians in Iraq where jihadists hold swathes of territory.
'Absurdity and horror'
Standing near the bucolic peak of Hartmannswillerkopf, Gauck reminded onlookers that the site "symbolises the absurdity and horror of those years".
"We commemorate the dead, the missing, the injured on both sides, and we honour their memory. They are not forgotten," he said.
The symbolism of the event was all the stronger as August 3, 1914 "opened a period of 30 years of conflicts, bitterness, massacres and barbarity between France and Germany," the French presidency said in a statement.
It testifies "to the strength of the friendship between the two countries which allows them to look together at their common history, including at what has been the most dramatic."
This is not the first time that Hollande and Gauck have joined forces to denounce the horrors of conflicts that once made enemies of the two countries.
Last year, the two heads of state walked hand in hand in the central French village of Oradour-sur-Glane where SS troops massacred 642 people on June 10, 1944 during World War II, the worst Nazi atrocity in occupied France.
In Hartmannswillerkopf, Hollande and Gauck also signed a joint declaration on Franco-German friendship as the foundation stone for the first World War I museum jointly conceived by historians from both countries was laid on the ground.
They then stood in silence in front of a monument under which are buried the ashes of some 12,000 unknown French and German soldiers.
Commemorations further afield
Other countries held events Sunday or were due this week to commemorate the 1914-1918 war, which left some 10 million dead and 20 million injured on the battlefields. Millions more perished under occupation through disease, hunger or deportation.
In London, an altar cloth embroidered by 138 wounded World War I soldiers was to take its place again at Saint Paul's Cathedral on Sunday.
The frontal, which has not been on display there for seven decades, was embroidered by severely wounded or shell-shocked men from Britain, Australia, Canada and South Africa in memory of their fallen comrades in arms.
And in Liege, a service to mark 100 years since the German invasion of Belgium will be attended Monday by members of the Belgian and British royal families as well as other heads of state and government, including Hollande and Gauck.