VERDUN, France (AP) — France's president and Germany's chancellor want their countries' improbable friendship to be a source of hope for today's fractured Europe as they commemorate the centenary of the longest battle of World War I.

In solemn ceremonies Sunday in the forests of eastern France, Francois Hollande of France and Angela Merkel of Germany are marking 100 years since the 10-month Battle of Verdun, which killed 163,000 French and 143,000 German soldiers and wounded hundreds of thousands of others.

Between February and December 1916, an estimated 60 million shells were fired in the battle. One out of four didn't explode, and remain in the soil. The front line villages destroyed in the fighting were never rebuilt. The battlefield zone still holds millions of unexploded shells, making the area so dangerous that housing and farming are still forbidden.

With no survivors left to remember, the commemoration now focuses on educating youth about the horrors and consequences of the war. Some 4,000 French and German children will take part in Sunday's events, which conclude at a mass grave where French President Francois Mitterrand took German Chancellor Helmut Kohl's hand in 1984 in a breakthrough moment of friendship and trust by longtime enemy nations.

Merkel said Saturday the event shows "how good relations between Germany and France are today" and the achievements of European unity.

"In a world with global challenges, it is important to keep developing this Europe," she said in a weekly address, expressing hope that Britain would not vote to leave the European Union in a June 23 referendum.

Amid rising support for far right parties and divisions among European countries over how to handle refugees, Hollande said he wants to work alongside Merkel to "relaunch the European ideal."

"We must take action ... at a moment when Europe is affected by the disease of populism," he told France Culture radio this week. He also noted the threat from violent extremism, saying the EU "must protect the people" especially against "terrorism."

Hollande and Merkel will spend the entire day together, starting at the cemetery of Consenvoye where 11,148 German soldiers are buried. Then they'll visit Verdun city hall to honor the martyred city, almost entirely in ruins at the end of the war.

After lunch, they will visit the newly renovated Verdun Memorial. The museum, which reopened in February, immerses people in the "hell of Verdun" through soldiers' belongings, documents and photos.

"The visit follows the steps of the soldiers. First reaching the front, moving into shell holes, fighting, surviving on the front line, the daily life," said historian Antoine Prost.

From the new rooftop of the museum, visitors can observe the battlefield site.

The main ceremony will take place in the afternoon at the Douaumont Ossuary, memorial to 130,000 unidentified French and German soldiers.

Verdun has become a common place of remembrance because "it's a place of massive death equivalent for the French and the Germans," Prost stressed.

The ceremony conceived by German filmmaker Volker Schlondorf will include children re-enacting battlefield scenes to the sound of drums amid thousands of white crosses marking graves.

Merkel, noting the importance of spreading the message of peace to younger generations, said, "I don't think you can shape the future if you don't also concern yourself with the past."