The Field of Remembrance has been organised by the Poppy Factory and held in the grounds of Westminster Abbey since November 1928, when only two Remembrance tribute crosses were planted.
Prince Harry, who has previously served in the British military, was pictured laying a cross at the memorial.
The small wooden memorial was placed surrounded by wreaths of poppies outside the ornate central London place of worship.
He was then followed by his wife, the Duchess of Sussex, who conducted the same routine.
Harry and Meghan then stood side-by-side as he saluted in memory of those who have lost their lives defending Britain in conflict around the globe.
More than 70,000 crosses of the kind laid by Harry are made by a small team based in south-west London every year.
In the run-up to Remembrance Sunday, members of the monarchy are attending a series of events commemorating those who paid the ultimate sacrifice, culminating in the service at the Cenotaph, where and senior royals will gather to pay their respects.