Two minutes' silence have been held to remember those who have fallen in battle.
Marked by the first stroke of Big Ben at 11am and the firing of a gun from Horse Guards Parade by The King's Troop Royal Horse Artillery, the silence at the Cenotaph in London was followed by The Last Post, sounded by the Buglers of the Royal Marines.
The Queen laid the first wreath in Whitehall, followed by the Duke of Edinburgh, the Duke of Cambridge, the Duke of York, the Earl of Wessex, the Princess Royal, Prince Michael of Kent and Field Marshal Lord Guthrie of Craigiebank.
Wreaths were also laid by Prime Minister David Cameron, Liberal Democrat leader Nick Clegg, opposition leader Ed Miliband and Westminster Plaid Cymru group leader Elfyn Llwyd, as well as high commissioners from Commonwealth countries and leaders of the Armed Forces.
Thousands of veterans have also marched through Whitehall to pay their respects at the Cenotaph.
The Prince of Wales and Duchess of Cornwall, currently on their Diamond Jubilee Tour in New Zealand, had already paid their respects, laying a wreath of poppies at the Auckland War Memorial Museum.
There are a number of services taking place, including at the National Memorial Arboretum in Staffordshire, which was attended by around 3,000 people.
The arboretum's focal point, the national Armed Forces Memorial, is designed so that on the 11th hour of the 11th day of the 11th month, a shaft of sunlight dissects its inner and outer walls, falling on a bronze wreath sculpture.
Services also took place at St Giles Cathedral in Edinburgh and Enniskillen in County Fermanagh.
For the first time, the two most senior members of the Irish government attended Remembrance events in Northern Ireland.
Irish premier Enda Kenny was in Enniskillen, 25 years after the 'Poppy Day bombing' which killed 11 people and injured more than 60. Mr Kenny laid a laurel wreath at the war memorial, only yards from where the no-warning blast detonated a quarter of a century ago
Deputy leader Eamon Gilmore also laid a wreath with Northern Ireland Secretary Theresa Villiers in Belfast.
In Lashkar Gah, Afghanistan, around 150 servicemen and women attended a service at 11am Afghan time, marking the two-minute silence before a bugler sounded the reveille.
Other ceremonies have been held across Helmand at patrol bases and checkpoints by the 9,500 British forces serving there.
Thousands of people also respected the two-minute silence on Twitter, abstaining from posting messages during the period of reflection. The idea was spread using the hashtag #2minutesilence.
On Saturday night, Rod Stewart performed for the Queen at the Royal British Legion's Festival of Remembrance in honour of Britain's war dead.
The show included the traditional two minute silence as poppy petals fell from the roof of the Royal Albert Hall, each representing a life lost in war.
Two minutes of silence begin at 11am every year on November 11 to commemorate the armistice signed between the Allies of World War One and Germany for a ceasefire on the Western Front, which took effect at the 11th hour of the 11th day of the 11th month of 1918.