The Queen joined 2,000 people at Westminster Abbey to mark the 90th anniversary of the Gallipoli landings.
More than 100,000 soldiers were killed during the eight-month campaign in Turkey.
Among those who died during the bid to capture the Turkish capital were 21,000 British and Irish troops.
Prince Charles, representing the Queen, attended a series of ceremonies, including a dawn service at the landing site.
He visited St Helles British cemetery, in Gallipoli peninsula, which is dedicated to the Commonwealth soldiers killed.
In Cyprus, soldiers from the Wiltshire-based 1st Royal Horse Artillery joined UN counterparts for a service.
The event at Wayne's Keep Cemetery took place at the exact time the first troops went ashore.