The Gallipoli campaign, part of a British-led effort to defeat the Ottoman Empire, aimed to secure a naval route through the Dardanelles from the Mediterranean Sea to Constantinople, now Istanbul, in Turkey. More than 100,000 troops died in the failed campaign that lasted into 1916.
At the Whitehall service, Reverend Dr Lyndon Drake recited from The Fallen by poet Laurence Binyon before a Royal Marines Portsmouth Road Band trumpeter played the last post after which there was a one-minute silence.
The high commissioners for New Zealand and Australia, Hamish Cooper and Jay Weatherill, walked in tandem to lay their own wreaths.
The princess joined other attendees in singing the hymn O God Our Help in Ages Past before men and women in military uniforms marched off Whitehall to the commemoration and thanksgiving service at nearby Westminster Abbey.
After that service, Catherine spoke with some of the military families.
