* World leaders join ceremony at Arc de Triomphe
* Macron leads tributes to fallen soldiers
* As he arrives, Putin gives Trump thumbs up (Adds White House comment)
By Richard Lough and Luke Baker
PARIS, Nov 11 (Reuters) - French President Emmanuel Macron led tributes to the millions of soldiers killed in World War One on Sunday, using an emotional ceremony in Paris attended by scores of world leaders to warn against nationalism a century on from the conflict.
U.S. President Donald Trump, Russian President Vladimir Putin, German Chancellor Angela Merkel and dozens of monarchs, princes, presidents and prime ministers joined Macron to mark the moment the guns fell silent across Europe 100 years earlier.
Under grey skies and gently falling rain, many heads of state joined Macron in walking the last stretch of the Champs Elysees to the Arc de Triomphe, where the ceremony was held. Trump and Putin arrived slightly later in separate motorcades.
In a 20-minute address delivered beside the tomb of the unknown soldier, Macron described the "unimaginable hell" of those who fought in the trenches, denouncing the nationalism that fanned the flames of war and now shows signs of resurgence.
Macron spoke bluntly of the threat from nationalism, calling it a betrayal of moral values. Trump, who has described himself as a nationalist and has promoted what he calls an "America First" policy, sat a few feet (meters) away, stony-faced.
"Patriotism is the exact opposite of nationalism: nationalism is a betrayal of patriotism," Macron said. "When we say 'our interests come first, those of others don't matter', we erase the very thing that a nation holds most precious, that which gives it life and makes it great: its moral values."
The commemoration marked the centrepiece of tributes to honour those who died during the 1914-18 war and to commemorate the signing of the Armistice that brought the fighting to an end at exactly 11 a.m. on Nov. 11, 1918.
"The lesson of the Great War cannot be that of resentment between peoples, nor should the past be forgotten," he said, alluding to the millions of women widowed and children orphaned by the conflict, as well as the 10 million soldiers killed. Around him, sorrow etched on their faces, former French soldiers stood rigidly to attention.
"It is our deeply rooted obligation to think of the future, and to consider what is essential," Macron said.
Under a glass canopy at the foot of the Arc de Triomphe, built by Emperor Napoleon in 1806, Trump, Merkel, Macron, Putin and the other leaders listened through earpieces as the French president spoke. Putin, who was last to arrive at the ceremony, gave Trump a brief thumb's up as he greeted them.
The 90-minute commemoration included the reading by children of letters written by German, French and British soldiers during the war, a recital by cellist Yo-Yo Ma and a moving performance of Maurice Ravel's Bolero.
"OLD DEMONS REAWAKENING"
After the ceremony, leaders returned to the Elysee presidential palace for a lunch hosted by Macron, while the spouses lunched at Versailles.
Over Britanny lobster and Bresse chicken, Trump, Putin, Macron and U.N. Secretary General Antonio Gutteres, sitting next to each other, were able to have a "rich, free-flowing discussion" about hot issues such as Iran, Syria, Saudi Arabia and North Korea, the Elysee said.
White House press secretary Sarah Sanders later said the discussion between Trump, Macron, Merkel, Putin and other world leaders also included the Intermediate-Range Nuclear Forces Treaty, a Cold War-era pact that Washington has said it plans to quit because Russia is violating it. Other topics, in addition to the ones listed by the French, were trade, sanctions, Afghanistan and China, she said.
"They had very good and productive discussions during the two-hour lunch," Sanders told reporters aboard Air Force One as Trump flew back to Washington.
In the afternoon, Trump travelled to the Suresnes American Cemetery just west of Paris to speak to veterans. He pulled out of a similar ceremony east of Paris on Saturday due to the weather.
"It is out duty to preserve the civilization they defended," Trump said of those who fought in the Great War. "We renew our sacred obligation to memorialize our fallen heroes" at the cemetery, "where they rest for all eternity," he said.
Sunday's events were the culmination for Macron of a week of commemorations of the war, one of the bloodiest in history and one that reshaped Europe's politics and demographics.
In the preceding days, Macron had toured former battlefields along France's western front, where he warned that nationalism threatened to undo the European unity so carefully rebuilt since World War Two - a theme he returned to on Sunday.
"Old demons are reawakening, ready to sow chaos and death," he said, warning of how ideology, religion and a disregard for facts could be exploited. "History sometimes threatens to repeat its tragic patterns, and undermine the legacy of peace we thought we had sealed with the blood of our ancestors."
On Saturday, in a rare public display of emotion by the leaders of two world powers, Macron and Merkel held hands on Saturday during a poignant ceremony in the Compiegne Forest, north of Paris, where French and German delegations signed the Armistice in 1918 that ended the war.
As Trump prepared to leave France, Macron opened the inaugural Paris Peace Forum, which seeks to promote a multilateral approach to security and governance and ultimately avoid the errors that led to the outbreak of World War One.
Merkel said the forum showed that "today there is a will, and I say this on behalf of Germany with full conviction, to do everything to bring a more peaceful order to the world, even though we know we still have much work to do." (Additional reporting by Steve Holland, Laurence Frost, John Irish and Michel Rose, Matt Spetalnick in Washington Editing by Matthew Mpoke Bigg and Sandra Maler)
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