Paris (AFP) - United in the rain, world leaders walked slowly up the Champs-Elysees in Paris to mark a century since the end of World War I -- but there were two notable absences, Donald Trump and Vladimir Putin.

The line of black-clad leaders was reminiscent of another moment of unity: after the 2015 Charlie Hebdo jihadist attack, when senior officials from more than 40 countries marched through Paris to denounce terrorism.

But on Sunday both the US and Russian presidents arrived separately for the solemn ceremony at the Arc de Triomphe, skipping the bus ride and symbolic walk with other leaders.

White House press secretary Sarah Sanders said Trump, who wore a cornflower -- France's war remembrance flower -- in his lapel, had arrived separately "due to security protocols".

His trademark bright red tie stood out among the dark coats in the front row as he and Putin stood either side of Macron.

The French president walked alone through the rain to greet veterans to the strains of the national anthem, La Marseillaise.

The moment stood in contrast to Trump's much criticised decision Saturday to call off a US cemetery visit because of rainy weather.

In another stark contrast with Trump, Macron went on to deliver a forceful speech against fear-mongering and isolationism, calling on fellow leaders to fight together for peace.

"Nationalism is a betrayal of patriotism," Macron said.

"Let us build our hopes rather than playing our fears against each other."

- 'Will we ever learn?' -

The ceremony, to remember the millions who died in the war, including colonial troops fighting for European powers, had a deliberately international flavour.

The superstar Chinese-American cellist Yo-Yo Ma performed alongside Beninese singer Angelique Kidjo and a European youth orchestra led by a Russian conductor.

High-school students read letters from soldiers at the front in English, French, German and Chinese.

"Am I dreaming?" read one letter from a French soldier upon learning that four years of brutal trench warfare were coming to an end in November 1918.

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"I'm so happy, I can hardly believe that the news is true," he wrote.

Another letter, from a British soldier, read: My darling parents, today has been perfectly wonderful. We got news of the armistice at 9.30 this morning... The streets were packed with wildly cheering civilians."

The crowds who braved the rain for the ceremony along the famed Champs-Elysees boulevard in Paris also came from far afield.

Nevan Lancaster, a 47-year-old from New Zealand, extended a work trip to Europe to pay his respects to a grandfather who fought in the war.

"I'm here for him. He didn't speak about the war -- it was his job, it was his duty," Lancaster told AFP.

Keith Evans, a 70-year-old Scot, said he had come "to remember the sacrifice, and the cost" of a conflict that claimed up to 20 million lives.

"I hope that with this event, the leaders here will learn the lesson of what President Macron has just said," Evans said, a remembrance poppy pinned to his raincoat.

"We should do better for peace," he added. "Will we ever learn? I hope so."

After the ceremony, which ended with Macron lighting anew the flame at the Arc de Triomphe's Tomb of the Unknown Soldier, the leaders headed for lunch at the presidential palace.

As Trump made his way towards his limousine, there was some booing from the crowd.