Queen Elizabeth was joined by the Duke of Edinburgh as well as political leaders for the event at the memorial in Whitehall, London. A two-minute silence took place at 11am GMT as wreaths are laid and thousands of veterans will march through Whitehall to remember their fallen comrades.
The service at the Cenotaph was shorter this year to cut the length of time veterans have to stand. But the plan to have political leaders lay wreaths together was dropped.
This year marks a number of other significant anniversaries in British military history, including the 75th anniversary of the Battle of Britain.
King Willem-Alexander of the Netherlands laid a wreath, after being invited by the Queen to mark the 70th anniversary of the liberation of his country following the end of the World War Two.
Falling poppies will be projected onto the Big Ben from dusk.
Commander Duncan McClement, from the RAF, told the BBC the country must not forget the personnel serving today in conflicts around the world today.
"There are also families for which this is the first Remembrance Sunday, who have lost loved ones on active service, and it's important that they, like all the others before through the great wars and all conflicts since then, are always remembered and never forgotten."
Sergeant Rick Clement, who lost his legs in Afghanistan, walked with his prosthetic limbs for the first time to lay a wreath in remembrance of four colleagues who lost their lives in the conflict.
"To take those few steps in their honour and all the others that paid the ultimate sacrifice, it will mean the world to me," he said.
In Scotland, landmarks and buildings were illuminated in red as part of the 2015 Scottish Poppy Appeal.
The question of whether Labour leader Jeremy Corbyn would wear a red poppy was answered after he was seen wearing one at last night's Festival of Remembrance at the Royal Albert Hall.