Hundreds of people pressed against a fence on Saturday to watch as Kate commemorated the 1915 Gallipoli landing of Australian and New Zealand troops in the First World War.
A woman in New Zealand military uniform handed the princess a wreath, which she placed at the foot of the national war memorial on Whitehall to mark when troops of the Australian and New Zealand Army Corps landed on the western shore of the Gallipoli peninsula on April 25 1915 as part of the failed campaign that lasted into 1916.
The ring of poppies with white flowers on top had a note signed Catherine and William that read: "In memory of the Australian and New Zealand soldiers who made the ultimate sacrifice for our freedom."
The high commissioners for New Zealand and Australia, Hamish Cooper and Jay Weatherill, then walked in tandem to lay their wreaths.
A Royal Marines Portsmouth Road Band trumpeter played the Last Post and a one-minute silence followed.
Kate joined attendees singing the hymn O God Our Help in Ages Past before the men and women in military uniforms marched off Whitehall.
The princess will join a commemoration and thanksgiving service at Westminster Abbey later on Saturday.
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.
Earlier on Saturday, the Princess Royal attended a dawn service at Wellington Arch at Hyde Park Corner in London.
Organised by the New Zealand and Australian high commissions, Anne arrived for the Anzac service shortly before it started at 5am.
She laid a wreath against Wellington Arch during a service that included a reading of the John McCrae poem In Flanders Fields and concluded with the national anthems of the United Kingdom, New Zealand and Australia.
Services were also held across New Zealand, Australia and on the Gallipoli peninsula in Turkey on Saturday morning.
The day was also marked in Villers-Bretonneux, a village in the Somme region of France, which Australian units helped defend during the World War I.
A post on the Royal Family X account on Saturday morning said: "Today is #ANZACDay - which honours the members of the Australian and New Zealand Army Corps (ANZAC) who served and died in all wars, conflicts and peacekeeping operations."
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