The nation is in mourning after the death of its longest serving monarch Queen Elizabeth II.
She had been head of state for the entire life of a huge proportion of people who live in the UK after she ascended to the throne on 6 February 1952.
Her 70-year reign represents the current era for millions of people, after they grew up and lived their lives with her always there, as a constant presence.
Flags on public buildings followed the example of the royal residences by being lowered to half-mast, shortly after 6.30pm.
People have been laying flowers outside royal residences in tribute to the 96-year-old, as they came to say "goodbye" and "thank you".
In Windsor, where a rainbow appeared as the news of the monarch's death was announced, the crowds outside the castle were so big that staff members struggled to drive through the main entrance.
A local Marks & Spencer kept its doors open late to allow mourners to buy bouquets, with one well-wisher reflecting on the likely thoughts of many when she said it felt "special" to gather with hundreds of others in the town which the Queen regarded as her home.
Dozens of colourful bunches of flowers were laid at the wrought iron gates of Buckingham Palace and a crowd of at least 1,000 had gathered within minutes of the announcement, prompting Royal Parks staff to begin constructing pens from metal barriers to control the public.
A series of spontaneous cheers and renditions of the National Anthem went through the crowd, remembering a head of state for which they felt nothing but respect, and many wept as they watched the Union Flag lowered above the building.
Numerous people left notes, including one which read: "We had you for longer than we deserved you."
Hundreds clamoured to catch a glimpse of the plaque announcing the monarch's death which officials displayed on the gates outside the palace.
One of those outside, Christine Evans, 68, from Shropshire, said the Queen was a "wonderful" leader who would be "sorely missed by us all".
Outside Buckingham Palace, Ms Evans said: "I'm proud to be here and to see all this support, and how loved the Queen is by all these people.
"She will be sorely missed by us all."
Another mourner in the crowd outside the palace, Annette German, added the Queen "stayed clear of all the arguments" in politics and "never put a foot wrong".
Hundreds streamed down the pavements winding through nearby Green Park on their way to join the crowds massing outside the palace.
Others visited Sandringham House on the Queen's Norfolk estate, where they laid flowers at the Norwich Gates.
Some appeared visibly upset as they approached the iron gateway and security workers closed the road to traffic, to ensure the growing crowds could safely gather.
At Hillsborough Castle, the official residence of the monarch in Northern Ireland, a stream of visitors laid tributes at the gates.
Other chose to visit churches across the nation, many of which had opened their doors to allow people to mark her death.
Dennis Edwards, joining those mourning at the Birmingham city centre cathedral, said it was "sad for the family, a sad day to the country".
He added: "Rest in peace, Queen Elizabeth, 70 years, seven years longer than (Queen) Victoria. You did well."
It came as churches were urged to toll their bells across England on Friday in order to mark the Queen's passing.
The House of Commons announced that it would sit at noon on Friday to allow MPs to pay tribute to the Queen in a session due to last until 10pm.
It added that there would also be a rare Saturday sitting, when senior MPs will take the oath to King Charles III from 2pm, with tributes continuing again until 10pm.
Around the country, electronic advertising hoardings switched to tributes to Her Majesty, with pictures of her life and the dates of her reign, with Piccadilly Circus's bright display boards among them.
The London Eye dimmed its lights and the Blackpool Illuminations were switched off as a mark of respect.
Messages were displayed on big screens at football grounds, where minutes' silences were also held, with matches involving West Ham, Arsenal and Manchester United among those where tributes were paid.
A meeting of governing bodies across a range of sports is understood to be scheduled for Friday morning, where officials from the Department of Digital, Culture, Media and Sport (DCMS) will set out government guidance on the official period of mourning.
TV networks changed their schedules to allow for a series of programmes to mark the life of Her Majesty, with several of the national networks showing wall to wall news coverage of the event.
The newspapers also all rushed out commemorative editions, marking the monarch's death and life.
Theatres across the country were due to dim their lights, observe a minute's silence, play the national anthem and open books of condolences before performances as mark of their respect, a statement from Society of London Theatre (SOLT) and UK Theatre said.
Black cabs lined the Mall under dark and rainy skies to pay tribute, with one of the cabbies, Michael Ackerman, explaining she was "our constant in our lives and we're showing our respect".
The Mercury Prize award ceremony, which had been scheduled for Thursday night, was postponed.
The Royal Society for the Prevention of Cruelty to Animals, of which the Queen was patron, said it would create a new medal that it would award in the sovereign's name as a tribute to her.
A series of strikes which had been planned for the coming days were called off, including industrial action by the TSSA, the RMT and Royal Mail workers.
At Balmoral Castle, where the Queen died, police removed barriers blocking the gates to allow the public to lay flowers, having earlier blocked them off as members of the Royal Family arrived.
One message read: "I love you Queen. I'll miss you. Tilly, aged 4."