THIEPVAL, France (AP) — One week after Britain's vote to leave the European Union, British Prime Minister David Cameron and members of the royal family were standing side-by-side with France's president to celebrate their historic alliance at the centenary of the deadliest battle of World War I.

More than 1 million people were killed, wounded or went missing in the Battle of the Somme in northern France, pitting British and French troops against German ones from July 1 to Nov. 18, 1916.

Britain held a moment of silence Friday morning to mark 100 years since the bloodiest day of British military history — about 20,000 British soldiers alone were killed on the first day of battle.

The main ceremony started with the sound of cannon shots shortly after noon Friday at the monumental Memorial of Thiepval in northern France with the participation of 600 British and French children. Each of them laid a flower crown on the 600 British and French graves of the cemetery.

Many descendants of soldiers, often wearing poppy and cornflower pins — the British and French symbols to remember those who died— were attending the event.

Guests and dignitaries, including French President Francois Hollande, Cameron and Britain's Prince Charles read texts describing the horrific scenes and the devastated landscapes of the front line in 1916.

The very solemn ceremony turned moving when French, British and Irish songs inspired by the war were sung.

The Memorial of Thiepval, built in 1932 by the British government, is dedicated to the 73,367 British and South African soldiers missing in the Somme area during World War I. Thousands of petals of poppies and cornflowers fell from the top of the monument in complete silence.

No speeches were programmed during the ceremony, yet Hollande and Cameron held brief informal talks following the event.

The two leaders wanted to seize the occasion to stress their World War I alliance and show their attachment to the ideas underpinning European unity.

Commenting on the British vote, Hollande told reporters the U.K. will "remain an ally and a partner of France."

"The decision has been made; it cannot be delayed or cancelled. Now we must draw the consequences," he said.

Both leaders agreed the relationship between Britain and France was "enduring and strong" and Cameron "reiterated his view that the United Kingdom should seek the closest possible relations with the EU and in that context, the need for constructive post-referendum negotiations," according to the prime minister's office.

Others at Friday's event included Prince Charles' wife Camilla, his sons Prince William and Prince Harry and William's wife Kate, Irish President Michael D. Higgins and former German President Horst Koehler.

British commuters were met by the eerie sight of WWI soldiers in uniforms as they made their way to work on Friday. Young men in vintage uniforms sat, stood and mingled with travelers at railway stations across the country during the morning rush hour. Some sang wartime songs, while others handed out cards bearing the names of the soldiers killed on the first day of the battle and the hashtag #wearehere.

Later Friday, Charles and Camilla were to pay tribute to Northern Irish and Canadian soldiers in two separate events in Thiepval and in the nearby village of Beaumont-Hamel.

Last month, Hollande and German Chancellor Angela Merkel commemorated together the centenary of the Battle of Verdun, the longest battle of World War I. The leaders praised their countries' friendship, risen from the ashes of two world wars and strengthened through EU cooperation.

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Greg Katz and Jill Lawless in London contributed.