Lights across Britain were switched off for an hour on Monday in a tribute to the dead of the First World War, inspired by the prophetic observation of then-Foreign Minister Lord Grey on the eve of war 100 years ago.

"The lamps are going out all over Europe," Grey said in 1914. "We shall not see them lit again in our lifetime."

British landmarks like Trafalgar Square were dark from 10 p.m. local time, and Prime Minister David Cameron asked all Britons to switch off all but a single light in their homes for an hour.

The "war to end all wars" spread carnage across Europe, especially northern France and Belgium, killing 17 million soldiers and civilians in 1914-18. One million of the dead were soldiers from Britain and its then empire, including 60,000 Canadians.

Cameron and Prince William, second-in-line to the throne, attended 100th anniversary ceremonies in Scotland and Belgium on Monday. Speaking at an event in Liege, William paid tribute to those who died as he noted that the current fighting in Ukraine showed instability continued to stalk Europe.

The National War Memorial in Ottawa commemorates the 60,000 Canadians killed in the First World War. (Sean Kilpatrick/Canadian Press)

We were enemies more than once in the last century and today we are friends and allies," the prince said, alluding to Germany and its cohorts in WW I and WW II.

"We salute those who died to give us our freedom. We will remember them," he told Belgium's King Philippe and other heads of state attending the Liege ceremony at the Allies' Memorial, near to where German troops invaded Belgium in the early hours of Aug. 4, 1914 — the event that brought Britain and its empire, including Canada, into the war.

Politicians and royalty from 83 countries, including presidents François Hollande of France and Joachim Gauck of Germany, were among those on hand in Liege while in Glasgow, Scotland, Cameron was joined by heir-to-the-throne Prince Charles at a centenary service.

Monday evening, British actors and clergy took part in a ceremony at Westminster Abbey in London.

Harper speaks in Ottawa

Canadian Prime Minister Stephen Harper commemorated the 100th anniversary in Ottawa, saying Canada's independence was "forged in the fires" of the conflict.

Prime Minister Stephen Harper pauses for a moment after he laid a wreath at the National War Memorial during a ceremony in Ottawa to mark the 100th anniversary of the start of the First World War. (Fred Chartrand/Canadian Press)

Speaking at the Canadian War Museum, Harper spoke of how the Great War decimated a generation of young Canadians.

"Yet amid the appalling loss, by any measure, Canada as a truly independent country was forged in the fires of the First World War.

"That is to say, when the great nations of the world gathered, we must never forget that our place at the table was not given to us — it was bought and paid for," Harper said, referring to a number of battles, including Passchendaele and Vimy Ridge, as well as Ypres​, where John McCrae wrote In Flanders Fields.

Earlier Monday, Harper placed a wreath at the tomb of the unknown soldier at the National War Memorial.

"It is a time to remember and honour the sacrifices and tremendous achievements of the more than 650,000 brave Canadians and Newfoundlanders who left their families and the comfort of their homes to serve their King and country, as well as to preserve the universal values of freedom, peace and democracy that we hold most dear," Harper said in an earlier statement.

French President Francois Hollande, left, embraces German President Joachim Gauck as they participate in a commemoration ceremony in Liege, Belgium to mark the 100th anniversary of the start of WW I. (Virgina Mayo/Associated Press)

"The dedication, courage and determination demonstrated by our brave soldiers, sailors and airmen, who stood shoulder-to-shoulder with like-minded allies to fight for what they believed in, resulted in Canada emerging as a proud, victorious nation with newfound standing in the world."

Gov. Gen. David Johnston said in a statement that this important centenary also allows Canadians an opportunity to honour the sacrifices of the estimated 425,000 Canadians who served overseas between 1914 and 1918.

"Their service contributed to the outcome of the war and to Canada's emergence as an independent nation," Johnston said.

"As Governor General and commander in chief of Canada, and as a father and grandfather, I encourage all Canadians to reflect upon the dedication of those who served in the First World War and to remember their sacrifices," he said.

The declaration of war came in a telegram from the British government to the then Governor General. By the time it was over, more than 60,000 Canadians were killed, and many thousands more returned home broken.

Jonathan Vance, a professor at Western University in London, said Canadians had an expectation in 1914 that they would be joining the battles.

Many, he said, even celebrated the fact, largely because they believed the war would be short-lived.

Not expected to last

The war wasn't expected to last long. But instead of weeks, the continent was plunged into hardship and misery for more than four years.

The Great War claimed an estimated 14 million lives, including five million civilians as well as nine million soldiers, sailors and airmen from 28 countries. At least seven million troops were left permanently disabled.

On Sunday, an intense hug between Gauck and Hollande during a remembrance ceremony in eastern France close to the German border sealed again the friendship between the two neighbours, who have become the cornerstones of the European Union.

Monday's ceremony in Liege was significant since the battle for the forts around the city meant the first delay for Germany's enveloping move through Belgium, the so-called Schlieffen Plan strategy to defeat France in a matter of weeks.

Liege held much longer than expected, and allowed the allied forces to gather strength and keep Germany at bay within dozens of kilometers of Paris.

Gauck called the German plan "hapless" and deplored German actions against civilians and cities its forces passed through during the early weeks of the war.

By the end of autumn 1914, both sides dug in, and from the early battles, the war quickly changed into trench warfare on the Western Front, with hundreds of thousands of casualties in a barren landscape where poison gas often wafted through the air.

The U.S. joined the allies against the German and Austro-Hungarian empires in 1917 and provided a decisive impetus to break the deadlock before the Nov. 11, 1918, armistice.