Earlier, millions of people joined in a two-minute silence at 11:00 GMT that marked the end of World War One on the 11th hour of 11 November 1918.
Prince Charles laid a wreath on behalf of the Queen at the Cenotaph in London.
Ten thousand people – including veterans and relatives of WW1 soldiers – marched past the monument.
Prime Minister Theresa May said those who fought in the war had made “immense sacrifices”.
Services and special events have been taking place across the country and the world.
More than 1,000 beacons will be lit at spots across the UK on Sunday evening. The lights are intended to symbolise the end of the darkness of war and a return to the lightness of peace.
The Queen was joined at the Westminster Abbey service by Prince Charles, the Duchess of Cornwall and the dukes and duchesses of Cambridge and Sussex.
Mrs May and German President Frank-Walter Steinmeier – who earlier became the first German leader to lay a wreath at the Cenotaph in a symbol of peace – were among the 2,000-strong congregation.
The Queen laid flowers at the grave of the Unknown Warrior, whose body was brought from France in 1920.
Mrs May gave a reading and the Dean of Westminster Dr John Hall prayed for a time when “aggression between peoples and nations is transformed into friendship and collaboration”.
Earlier on Sunday, the Queen observed the two-minute silence at 11:00 in Whitehall from the balcony of the nearby Foreign and Commonwealth Office. The Duke of Edinburgh, however, was absent.
Prince Charles, the Duke of Cambridge and the Duke of Sussex, Mrs May, and Labour leader Jeremy Corbyn laid wreaths.
Big Ben’s renovations were paused ahead of Armistice Day to allow the bell to chime before and after the traditional two minutes of silence.
At 12:30, 10,000 members of the public – chosen by ballot – began a procession past the Cenotaph, beginning at the Mall and ending at St James’s Park at 13:30.
The march was described by organisers as a “nation’s thank you” to all those who fought in WW1, and featured descendants of veterans from across the country.
Emma Silk, 47, from Wiltshire, told the BBC she was proud and emotional to be walking in the parade.
“I want to honour the memory of my grandfather – what he did for our freedom,” she said.
Her grandfather lied about his age to join the war in 1917.
Helen Wightman, 67, from Surrey, was in London for the parade with her younger sister, Julie Jones, 53, from Wigan, to remember their grandfather, Charles Izzard.
“I’ve got six grandchildren and think its important that we remember if it was not for them, we would not have the freedom that we have today,” Helen told the BBC.
They were wearing his medals but details of his experience are hazy. Julie did try to interview him once for a school project but he got too upset.
“So many men died, he thought he was one of the very lucky ones,” she said.
Jeremy Cripps, 64 and from South Shields, walked in the parade in honour of his grandfather, Robert Malin.
Mr Cripps said his grandfather went beyond the call of duty by going out under fire to rescue wounded soldiers.
The Mall was packed with people proud to remember the part their families played in the war.
Dressed in the sombre, dark clothes of remembrance, many wore an ancestor’s solitary medal, others row upon row, alongside their poppies.
They carried wreaths with carefully crafted messages of thanks and spoke of their pride in their grandfathers, great-grandfathers and great uncles.
In France, where many of the battles of the Western Front were fought, 70 world leaders gathered for a ceremony at the Arc de Triomphe in Paris, hosted by French President Emmanuel Macron.
In a speech, Mr Macron urged the assembled world leaders to come together in a joint “fight for peace”.
“Let us build our hopes rather than playing our fears against each other,” he said, describing patriotism as “the exact opposite of nationalism”.
Remembrance services were also held at Llandaff Cathedral in Cardiff, Glasgow Cathedral, St Anne’s Cathedral in Belfast.
On the eve of Armistice Day, the Queen and senior royals attended the annual Festival of Remembrance at the Royal Albert Hall.
The royals stood as thousands of people in the audience held up photos of World War One soldiers – including Mrs May and husband Philip who held pictures of two of her ancestors.
Armistice Day, 1918
Armistice Day falls each year on 11 November to mark the day in 1918 when the fighting in World War One was stopped.
The Allies and Germany signed an armistice in a railway carriage in the Forest of Compiegne in at 05:00. Six hours later, at 11:00, the conflict ceased.
King George V announced that a two-minute silence would be observed in 1919, four days before the first anniversary of Armistice Day. The silence continues to be observed every year on the 11th hour of the 11th day of the 11th month.