British Prime Minister David Cameron and his wife, Samantha Cameron, visit the Tower of London, where they each lay a poppy at the art installation "Blood Swept Lands and Seas of Red" by artist Paul Cummins, which marks the centenary of the World War I on Saturday in London, England. Photo: CFP

Queen Elizabeth II led Britain in paying silent tribute to the Commonwealth war dead on Remembrance Sunday, an annual event made particularly poignant this year on the centenary of the start of World War I.

The 88-year-old monarch, senior royals and politicians including Prime Minister David Cameron laid wreaths at the Cenotaph national war memorial in London, as hundreds of veterans from more than 70 years of conflicts looked on.

Security was tighter than normal amid heightened fears of the risk of a terror attack, but there was no change to the customary program of marches and military music.

A 13-pounder World War I gun was fired at 1100 GMT, marking the start of two minute's silence observed by millions of people across Britain and at British military bases across the world.

Remembrance Sunday is the Sunday closest to Armistice Day on November 11, the anniversary of the 1918 signing of the peace that ended fighting in World War I. More than 1 million people from the British empire died in the four-year conflict, but the day has become a time to remember all the troops killed in wars since then.

It is thought there has been only one year - 1968 - without a British military fatality on active service since the end of World War II in 1945.

Cameron said the ceremonies were particularly poignant as 2014 is the centenary of the start of World War I, as well as the 70th anniversary of the D-Day landings and the end of Britain's combat role in Afghanistan.

The queen laid the first wreath followed by her husband Prince Philip, 93, her son and heir Prince Charles and grandson Prince William, all of whom have served in the military.

Forty-six high commissioners from Commonwealth countries then each laid a wreath.

And in a sign of the improved relations between Ireland and Britain, the Irish ambassador laid a wreath for the first time in honor of thousands of Irishmen who died in British uniform.

Meanwhile Cameron has launched plans for a 1 million pounds ($1.6 million) national memorial to the more than 220,000 troops who fought in Iraq and Afghanistan since 2001, 632 of whom died.

In the run-up to Remembrance Sunday, many Britons wear a paper red poppy symbolizing the flowers which grew on French and Belgian battlefields during World War I.

This year, the moat of the Tower of London has been filled with 888,246 ceramic poppies in a striking art exhibit to mark every single British soldier who died during that conflict.

An estimated 4 million people will see the installation, called "Blood Swept Lands And Seas Of Red", before it is taken down at the end of November.