Large crowds had already gathered on Sunday morning, and, by the afternoon, roads in the town were closed and walkways were packed with people.
Local churches also paid tribute to the Queen during Sunday worship, with one close to Windsor Castle set to hold a special service later in the day.
He was then formally proclaimed King in a historic ceremony at St James’s Palace in London following a meeting of the Accession Council, during which he swore an oath to privy counsellors.
A sea of flowers, cards and tributes were being placed outside the gates of Windsor Castle ahead of the local proclamation of the monarch near the Queen Victoria statue in the Berkshire town.
Stewards had been trying to divert mourners on a different route to the gates as a large crowd was building up.
Roads were also closed off.
Military veterans could be seen among those paying tribute.
Alvie Porter-Smith, 87, who lives nearby, speculated that the number of people visiting on Sunday could be the largest yet.
He told the PA news agency: “Already there are queues along the Long Walk, but it’s a rather nice atmosphere, with people making conversation.”
Also among the visitors was Lucy Pursley, from nearby Maidenhead, with her mother Kathy and three children, who took five bouquets of different-coloured flowers which they had each picked out at a nearby shop.
Ms Pursley said: “We really wanted to come here and show our respects. Each of us picked our flowers out and have written a message on them.
“It would be nice to know someone from the royal family read them just so they know what the Queen meant to everyone, even children.”
The next generation will not forget our great Queen
Ned Hart, from Egham, Surrey
Stuffed toys, cards, letters and gift bags were also seen among the tributes.
Ned Hart, 43, from Egham in Surrey, said: “I’ve been here every day and have laid flowers each time.
“It’s nice to see the the pile of tributes building up; I hope the whole area is covered by the time of the funeral.
“The number of children and young people I’ve seen here is also heart-warming, the next generation will not forget our great Queen.”
Windsor Parish Church of St John the Baptist will hold a special service for the Queen at 5pm.
Local churches paid tribute to the former monarch and spoke about the next chapter under King Charles III earlier in the day.
Reverend Canon Sally Lodge, speaking during a Sunday service at the Parish Church of St Stephen & St Agnes, said: “Gracious God, we give thanks for the life of your servant Queen Elizabeth, for her faith and her dedication to duty.
“Bless our nation as we mourn her death and may her example continue to inspire us.”
She went on to praise the “dedication to public service” of King Charles III.
Members of the church had the opportunity to sign a book of condolence and will sing the national anthem after the service ends.