French President Emmanuel Macron has urged world leaders marking the centenary of the World War One Armistice to reject nationalism.
Addressing leaders in Paris - including US President Donald Trump and Russian President Vladimir Putin - he described it as a "betrayal of patriotism".
"By saying 'our interests first and never mind the others' you stamp out the most precious thing a nation has - its moral values," he said.
Events are taking place worldwide.
Some 9.7 million soldiers and 10 million civilians died in World War One from 1914 to 1918.
Several world leaders also held bilateral meetings at the events. Mr Putin told journalists he had a brief conversation with Mr Trump and that it went well.
However, the French organisers of the lunch event changed the lunchtime seating arrangement at the last minute so Mr Trump and Mr Putin would not be sitting next to each other, Russian media reported.
What happened in Paris?
Mr Macron and dignitaries walked to the Tomb of the Unknown Soldier, a memorial to France's fallen under the Arc de Triomphe, in the rain under black umbrellas as church bells tolled through the city.
In a speech lasting nearly 20 minutes, the French leader called on fellow leaders to "fight for peace".
"Ruining this hope with a fascination for withdrawal, violence or domination would be a mistake for which future generations would rightly find us responsible," he said.
The service ended with the bugle call that was played at 11:00 on 11 November 1918 to signal the end of hostilities.
On Sunday afternoon Mr Macron and German Chancellor Angela Merkel attended a peace conference - the Paris Peace Forum - with leaders including Mr Putin and Turkey's Recep Tayyip Erdogan.
Mrs Merkel warned that "blinkered" nationalism was gaining ground in Europe and elsewhere.
On Saturday Mr Macron and Mrs Merkel visited the town of Compiègne, north of Paris. They signed a book of remembrance in a railway carriage identical to the one in which the 1918 Armistice was sealed.
What has Trump been doing?
Mr Trump did not attend the peace conference and left for the US shortly after it began.
Earlier he visited a cemetery in Suresnes in western Paris, saying he had gone there "to pay tribute to brave Americans" who died in the war.
He caused controversy on Saturday by cancelling a trip to another cemetery for the war dead because of bad weather.
Earlier on Sunday, just before the leaders assembled, a topless female protester with the words "fake peacemaker" written on her chest came within a few metres of the US president's motorcade before being apprehended.
A group of about 50 activist groups plan to hold a protest in Paris against his visit later.