Prince Charles laid a wreath on behalf of the Queen, followed by other Royals and prominent politicians - including Johnson.
A two-minute silence was held as part of the Remembrance Sunday commemorations.
They stood with heads bowed as Big Ben tolled 11am, and a two-minutes' silence was marked by the firing of a gun by the King's Troop Royal Horse Artillery and buglers sounded the Last Post.More news: Manchester United win 'template' for future performances - Solskjaer
The Duke of Cambridge, Duke of Sussex, Duke of York, the Earl of Wessex, the Princess Royal and the Duke of Kent all laid wreaths at the base of the memorial.
Kimberley Durrant, the director of the governments London office, took part in the National Service of Remembrance at the Cenotaph in the United Kingdom capital.
After wreaths are laid, the Bishop of London, Dame Sarah Mullally, will lead a service of remembrance which will end with trumpeters of the Royal Air Force sounding Rouse (Reveille).
He said: 'We remember the many fearless people from Britain and all across the world who put their lives on the line making huge sacrifices in two world wars which cost the lives of millions, and in all other conflicts since.More news: 'With Leo, anything is possible' - Valverde hails Messi
The Taoiseach has attended the Royal British Legion's Remembrance Day ceremony in Enniskillen, Co Fermanagh.
An equerry laid a wreath for the Duke of Edinburgh, who was not present after retiring from royal duties two years ago.
This was the 100th ceremony at the Cenotaph, following a tradition first begun after the first world war when thousands flooded into London and the memorial became a focal point for the bereaved.
The Festival commemorates all those who have lost their lives in conflicts, and this year will mark the 75th anniversary of a number of crucial battles of 1944 - Monte Cassino, Kohima and Imphal, and D-Day - with a particular emphasis on the role of the Commonwealth and Allied Forces.More news: Elisabeth Moss Is Fighting for Her Life