Services have taken place across the country to honour all those who have died in conflict.
The Queen led the commemorations at the Cenotaph in London. Prime Minister Theresa May and party leaders attended.
The Queen laid the first wreath, followed by other royals and politicians. After a remembrance service, veterans and current military personnel paraded down Whitehall.
More than 700 soldiers, sailors and members of the RAF lined Whitehall to hear a service led by the Bishop of London, the Right Reverend Richard Chartres.
Detachments from units such as the Royal Marines, Household Cavalry and Royal Gurkha Rifles flanked the stone memorial.
Scotland's First Minister Nicola Sturgeon attended the annual wreath laying and Remembrance Sunday service at St Giles' Cathedral in Edinburgh.
She said it was "an opportunity for all of Scotland to join with others right across the world and commemorate those who made the ultimate sacrifice in conflicts during the last century and more recently".
BBC correspondent Daniela Relph at Whitehall
The crowds are 20-people deep in places on Whitehall.
Those with a front row position had to start queuing to get through security at first light. People were craning their necks to catch a glimpse of the march past.
Whitehall today is a place of shared remembrance.
There are those who've come to the Cenotaph on Remembrance Sunday for the first time. And there are those for whom this is an annual event, something they would never miss.
They have come from all over Britain and beyond. Those I spoke to talked of feeling humbled during the 11 o'clock silence.
They also said that that being here, in person, was a really moving experience.
There are 8,000 people taking part in the march past. Watching it stirs contradictory feelings - it is both a solemn experience and somehow uplifting.
No-one here on Whitehall today will ever forget.
The Irish Prime Minister and Northern Ireland's First Minister attended a Remembrance Sunday ceremony in Enniskillen, Fermanagh.
And there was a National Service of Remembrance for Wales at Cathays Park in Cardiff.
First Minister Carwyn Jones said: "It is important we take the opportunity to pay tribute to those who lost their lives, paying the ultimate price for our freedom.
"Those who fought bravely for our futures must never be forgotten."
Elsewhere, more than 40,000 knitted poppies have been placed on a Remembrance Day parade route in Thirsk, North Yorkshire.
The organisers said volunteers had sent in knitted and crocheted flowers from all over the world.
In Croydon, where seven people were killed and 50 injured after a tram derailed earlier this week, special prayers are being said for those affected.
Remembrance Sunday
The prime minister has said the day is not only about honouring the dead but also for paying tribute to those members of the armed forces currently on active service combating so-called Islamic State.
Mrs May said: "The way of life we enjoy today depends upon the service offered by members of the armed forces and their families.
"Across generations, and in every corner of the UK, today we remember those who gave so much for our values, our democracy, and our nation."
The Imperial War Museums are releasing letters, photographs and memoirs donated by families of soldiers killed in the Battle of the Somme in 1916, the bloodiest clash in World War One.
The documents originally formed part of the museum's Bond of Sacrifice collection dating from its formation in 1917.
Contributions included a photograph of two brothers who it is said were within 50 metres of each other when one was wounded and the other rushed to his aid and was shot.
They died in each other's arms, and their mother received one telegram in the morning informing her of one son's death, and later the same day, another telegram telling her the other son had died.
The documents will now be part of the Lives of the First World War digital memorial.
At the Royal Albert Hall on Saturday evening, the Queen and other members of the Royal Family stood to applaud a procession of bereaved families.
It came after the audience heard the story of Cyrus Thatcher, who was killed in 2009 aged 19 while fighting as a rifleman in Afghanistan.
The Queen and the rest of the royal box joined the standing ovation as his parents, Helena Tym and Robin Thatcher, led a procession of bereaved families through the hall.
This year the event marked the centenaries of the Battle of the Somme and the Battle of Jutland, the 25th anniversary of the Gulf War and the 80th anniversary of the first flight of the Supermarine Spitfire.
Thousands of poppies fluttered to the ground from the hall's domed roof as the room observed a two-minute silence.
A two-minute silence has been held to mark Remembrance Sunday.