Prime Minister Gordon Brown, Foreign Secretary David Miliband, opposition party leaders and Commonwealth envoys also laid wreaths.
For the first time, the Territorial Army - which is celebrating its 100th anniversary - laid a wreath at the monument.
Royal British Legion spokesman Stuart Gendall said it was important for the nation to come together and remember all those that died fighting for their country.
'Pause for thought'
Remembrance services have been held in towns and cities across the UK.
Scotland's First Minister Alex Salmond laid a wreath at the Stone of Remembrance in Edinburgh and said this was a time to "pause for thought".
At the Welsh National War Memorial in Cathays Park, Cardiff, First Minister Rhodri Morgan paid tribute to the sacrifices made by members of the armed forces.
In Belfast, up to 1,000 people gathered in the grounds of the City Hall for the Act of Remembrance ceremony, which included a fly-past by three helicopters.
Des Feely, the father of the first female British soldier killed in Afghanistan, laid a wreath in her honour at a ceremony in Carlisle's city centre.
Corporal Sarah Bryant, 26, was killed on duty in June in a roadside explosion near Lashkar Gah, in the south of the country.
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Remembrance services in London, Iraq and Afghanistan
Mr Feely said: "It's important to keep remembering, and it's not just limited to the First or Second World War. There are ongoing sustained casualties in Iraq and Afghanistan.
"It highlights the fact that for people here in this country, their way of life - freedom of speech, freedom of movement - is still being preserved through the actions of others."
In Somerset, Harry Patch, Britain's last living veteran of the First World War trenches, laid a wreath in honour of his fallen comrades.
My father died aged 98 but on his death bed he called out for his horse Maisie, who saved his life on many occasions
"It was 90 years ago. But you can't forget it," said the 110 year old.
On the other side of the country, a lone Spitfire flew over Duxford near Cambridge, home to one of England's most famous Battle of Britain air bases.
In the Essex garrison town of Colchester, thousands gathered to salute the war dead, turning out in higher numbers than last year.
The Colchester-based 2nd Battalion, The Parachute Regiment, lost nine soldiers during its recent tour of Afghanistan, plus a further six linked to it.
Overseas services
British troops stationed at Afghanistan's largest military base, in Kandahar, held their own Remembrance Day ceremony.
The BBC's Ian Pannell in Afghanistan said in the 12 months since the last Remembrance Sunday service, 39 British men and women have died while serving their country.
Their comrades stationed in the southern Iraqi city of Basra also laid wreaths and attended a prayer service.
Harry Patch, accompanied by Henry Allingham, 112, and Bill Stone, 108 - the three remaining First World War veterans who still live in the UK - are due to mark the two-minute silence at the Cenotaph to commemorate Armistice Day on Tuesday.
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