Ceremonies have taken place at military bases, town halls, churches, schools, and at the National Memorial Arboretum in Alrewas, Staffordshire.
The Royal British Legion held its own event in London' s Trafalgar Square.
Meanwhile, the Duke of Edinburgh has visited Belgium, scene of some of World War One's deadliest battles.
This year, Armistice Day, which honours members of the armed forces who have died in that war and in all conflicts since, comes a day after Remembrance Sunday.
In commemorations across the UK:
Mairtin O Muilleoir became the first Sinn Fein Lord Mayor to attend an Armistice Day ceremony - at Belfast City Hall Welsh Secretary David Jones struck a £5 Remembrance Day coin at the Royal Mint, Llantrisant, near Cardiff At Glasgow's Central Station, a new memorial stone dedicated to railway staff who died in wars was unveiled In Wrexham, an air raid siren marked the start of the silence - a week ahead of A Company 3rd Battalion The Royal Welsh pulling out of the town's Hightown barracks In Staffordshire, 93-year-old Dorothy Ellis - thought to be the last surviving widow of a World War One veteran - joined senior representatives of the government and the armed forces at the National Memorial ArboretumThe arboretum's Portland stone 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.
It is the nation's tribute to more than 16,000 servicemen and women who have died on duty, or as a result of terrorism, since 1948.
Soldiers at military bases including Catterick Garrison, north Yorkshire, stood silent The Duke of Edinburgh visited Ypres - his first foreign trip since abdominal surgery in June. Silences were observed at memorials including the National Memorial Arboretum And those at the Cenotaph, Whitehall, were silent for a second day running At Central Station, Glasgow, a black granite memorial to war dead was unveiled The Royal British Legion's Silence in the Square is in its sixth yearIn London, the Royal British Legion's sixth Silence in the Square remembrance ended with members of the public dropping poppy petals into the Trafalgar Square fountains.
Actor Adrian Lester and Britain's Got Talent winner Paul Potts also appeared at the event alongside The Poppy Girls - five schoolgirls aged between 10 and 17 whose fathers are all servicemen.
Their song The Call (No Need To Say Goodbye) was released on Saturday as part of this year's Royal British Legion appeal.
In Lytham St Annes, Lancashire, meanwhile, hundreds of people from across the UK answered a request to attend the funeral of a World War Two veteran they never knew.
Harold Jellicoe Percival served as ground crew on the famous Dambusters raids carried out in May 1943 by 617 Squadron.
The funeral home organising the service put an advert in a local newspaper - which was then widely publicised on social media sites - appealing for people to attend.
Royal visitThe two-minute silence takes place annually at the 11th hour of the 11th day of the 11th month - the time the guns fell silent along the Western Front in 1918, and an armistice was declared.
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Armistice Day marked at the Menin Gate memorial
It was first observed in November 1919 following a suggestion by an Australian journalist.
Continue reading the main story At the scene Chris Morris BBC News, BelgiumIt was a bitterly cold day in Flanders for the launch of five years of commemorative events, marking the centenary of World War One. Crowds gathered at the Menin Gate in Ypres, where the names of more than 50,000 soldiers who died but have no known graves are inscribed on the walls.
With the Duke of Edinburgh in attendance on his first foreign trip since his abdominal surgery in June, soldiers and schoolchildren from the UK and Belgium carried 70 sandbags of soil, from local cemeteries, to a gun carriage of the Royal Horse Artillery. The soil will arrive in London at the end of this month to create 2013 black friday a memorial garden at Wellington Barracks.
A series of big memorial events are planned over the next few years marking key events of the war. The idea is to commemorate rather than celebrate, to create a far better understanding of the causes, conduct and consequences of a war in which an estimated 20 million soldiers and civilians died.
The proposal was supported by a former high commissioner to South Africa and endorsed by the cabinet and King George V just a few days before the first anniversary of the armistice.
In Belgium, Prince Philip, 92, attended a "sacred soil" ceremony in Ypres alongside soldiers of the Household Division, Belgian soldiers and British and Belgian schoolchildren.
Seventy sandbags filled with soil were collected from the World War One battlefields of Flanders by Belgian schoolchildren, the first time this has been allowed.
The soil, taken from Commonwealth cemeteries, will be placed in the Flanders Field Memorial Garden, due to open next year at The Guards Museum in London.
The trip was the duke's first outside the UK since undergoing abdominal surgery in June.
On Sunday, two-minute silences took place at war memorials across the UK and the Commonwealth.
In London, the Queen and other members of the Royal Family laid the first wreaths at the Cenotaph in Whitehall, before a march-past by 10,000 military veterans and civilian representatives.