King Charles has led the National Service of Remembrance at the Cenotaph in central London.
The service honours the contribution of military and civilian servicemen and women in World Wars One and Two, and other conflicts.
Thousands of veterans, army personnel and members of the public in the capital joined the rest of the UK in a two-minute silence at 11:00 GMT.
Members of the Royal Family and politicians also laid wreaths.
At 11:00 Big Ben chimed to signal the start of the two-minute silence. It was ended by the sound of a cannon blasting from Horse Guards Parade followed by a bugler playing the Last Post.
King Charles laid the first wreath on behalf of the UK and he saluted the Cenotaph before pacing back.
The second wreath was laid on behalf of Queen Camilla and this was followed by the Prince of Wales, the Duke of Edinburgh and Princess Anne who all stepped forward with their wreaths.
Prime Minister Rishi Sunak, Labour leader Sir Keir Starmer, members of the Cabinet and other political party leaders also took part in the first formal part of the service.
Queen Camilla viewed the Remembrance Day service from the Foreign Office balcony, alongside Catherine, the Princess of Wales.
Massed Bands and Pipers assembled and played in the rain at the Cenotaph as members of the armed forces laid their wreaths.
The Cenotaph remains under a dedicated 24-hour police presence until the conclusion of remembrance events.
In Scotland, First Minister Humza Yousaf laid a wreath at the Stone of Remembrance while Livingston and Rangers held a minute silence before their Scottish Premiership clash in West Lothian.
On Saturday, nine police officers were injured as they prevented a violent crowd of mainly football hooligans reaching the war memorial while a service took place.
Scuffles first broke out shortly after 10:00 GMT as police attempted to stop a crowd of people carrying St George's flags marching along Embankment towards Whitehall where the Cenotaph is located.
Officers faced aggression from some of the counter-protesters - who include some far-right groups - near the Cenotaph, the Metropolitan Police said.
The group, which had been chanting "England 'til I die", pushed through the police barrier, with some shouting "let's have them".
However, a two-minute silence held at the Cenotaph at 11:00 GMT to mark Armistice Day was observed "respectfully", police added.
No major protest is scheduled to take place on Remembrance Sunday, although the policing operation will continue with some 1,375 officers deployed amid commemoration events in the capital.
A parade through Whitehall followed the wreath-laying.
Among those marching are nuclear test veterans, who for the first time are wearing a medal acknowledging their contribution.
After 70 years of waiting for recognition, those exposed to the effects of nuclear bombs during the UK's testing programme were given a medal - depicting an atom surrounded by olive branches - for the Remembrance Sunday service.
More than 300 different Armed Forces and civilian organisations are represented in the march, as well as some 300 veterans not affiliated with an association who have been invited to join for the first time.
Earlier, Royal British Legion's Anthony Metcalfe told BBC Breakfast he expected "close to 10,000 individuals" to take part.
"Everyone from a 100-year-old D-Day veteran to young Scottish soldiers and many regimental associations and other groups," he said.