Those queueing say they wouldn't dream of missing the chance to bid farewell to Her Majesty - ITV News' James Mates reports.
Mourners are queueing into the night for a chance to pay tribute to Her Majesty, as the Queen's Lying in State opens to the public for the first time.
Mourners are slowly and silent filing past Her Majesty's coffin - some in tears - as thousands get one final opportunity to say farewell before her funeral on Monday.
Vast crowds gathered to witness first-hand the moment Her Majesty left Buckingham Palace for the very last time.
One mourner waiting at The Mall to witness the historic moment told ITV News: "I've been crying for days, and I know that when the gun carriage starts passing, I'll probably be a wreck, but it's okay."
As people settle in for a night of queuing friendships are being made, Shehab Khan talks to Brits waiting to pay tribute to the Queen.
Another added: "It's like a weird emotion keeps coming over you. One minute you're smiling at the memories, and then you're crying because it's a big loss."
As day turned to night, the queues were snaking across central London to the ancient hall, where hundreds of thousands are expected to pay their respects until the Queen's funeral on Monday.
King Charles III and his sons walked behind Her Majesty’s coffin during the procession to the hall, where many people have braved the queue for hours - with some even having camped out overnight.
People flew in from around the world, and travelled across the country to gather along the Mall, Whitehall and at Westminster for the once-in-a-lifetime opportunity to watch the Queen's casket pass by.
Her Majesty's children, the King, the Princess Royal, the Duke of York and the Earl of Wessex, accompanied her coffin on foot.
In the row behind were the Prince of Wales, the Duke of Sussex and Peter Phillips, the Princess Royal’s son.
The last time Prince William and Prince Harry stood side-by-side in a royal coffin procession was behind the casket of their great-grandmother, the Queen Mother.
As children, they stood side-by-side as they watched their mother, Princess Diana’s, coffin pass by.
The brothers were today followed by the Queen’s nephew, the Earl of Snowdown, her cousin, the Duke of Gloucester, and her son-in-law, Vice Admiral Sir Tim Laurence.
The Queen Consort, the Princess of Wales - who was wearing a brooch that belonged to Her Majesty - travelled behind the royals in the same vehicle.
The Duchess of Sussex and the Countess of Wessex followed in a separate car as they all journeyed to the Palace of Westminster, where the Queen will lie in state for four days before her funeral on Monday.
Placed on top of Her Majesty’s casket, draped in the Royal Standard, was the Imperial State Crown on top of a velvet cushion along with a wreath of flowers.
The wreath included white flowers along with foliage from the royal residences – pine from the gardens of Balmoral Castle, and lavender, rosemary and pittosporum from Windsor’s grounds.
As the coffin made its way to the historic Palace of Westminster, Big Ben tolled at one-minute intervals, while gun salutes were fired from Hyde Park by the Troop Royal Horse Artillery on the minute.
"She did not reign over us, she reigned for us", children's author Michael Morpurgo remembers the Queen with a special poem written for ITV News.
A number of people could be seen wiping away tears while many held up their phones to film as they watched the Queen's coffin pass by.
At one point, the procession poignantly passed by the statue of the Queen’s parents King George VI and the Queen Mother which overlooks The Mall.
Thousands spilled into public spaces across London, such as Hyde Park to watch the procession on large screens, while windows and balconies along Parliament Street were also filled with onlookers.
London’s City Hall confirmed that all dedicated ceremonial viewing areas for the procession reached capacity.
The Queen's coffin left in procession from Buckingham Palace at 2.22pm and arrived at Westminster Hall to cheers from crowds gathered around the area.
The King and the senior royals saluted the coffin as it was carried by a bearer party – eight soldiers from Queen’s Company 1st Battalion Grenadier Guards – into the Hall.
A short service was then led by the Archbishop of Canterbury accompanied by the Dean of Westminster.
The King and members of the royal family were present, as well as officials from the House of Lords, House of Commons, devolved Parliaments and Assemblies, along with the High Commissioners of the Realms.
Cries of “God save the King” could be heard as the King and the Queen Consort left Westminster Hall marking the end of the procession.
Four officers from the Household Cavalry – two from the Life Guards and two from the Blues Royals – have now begun the first six-hour vigil around the coffin, taking their places at the corner of the catafalque.
A double tap on the floor from the stick of the Officer of the Watch, who has command of the rotations, signalled the start of the vigil.
Mourners will be able to file past the Queen's coffin when her Lying in State opens to the public from 5pm, continuing until 6.30am on Monday, September 19 - the day of her funeral.
Officials expect the queue to stretch for about four miles along the River Thames.
It starts on the Albert Embankment by Lambeth Bridge and will continue along the South Bank to Southwark park in Bermondsey, south east London.
ITV News understands mourners could be waiting more than 30 hours and stewards could close the queue early if it gets too long.
A government spokesperson for the department handling the queue logistics - codenamed Operation Feather - told ITV News that stewards may have to turn people away if they are unlikely to get into the Hall before the Queen's coffin is moved for her funeral.
However, live updates will be provided regularly online showing wait times and detailing how how far back the queue is stretching.
The government has issued strict guidance on how mourners should behave inside the Palace of Westminster and urged them to check the list of prohibited items to "plan ahead and prepare appropriately".
"I'll always remember her absolute devotion and discipline" - Queen Rania of Jordan hails the Queen's dedication in an interview with ITV News presenter Mary Nightingale.
Ahead of the procession on Wednesday afternoon, the White House said US President Joe Biden had spoken with the King to "offer his condolences" and conveyed his wish "to continue a close relationship" with him.
"The President recalled fondly the Queen’s kindness and hospitality, including when she hosted him and the First Lady at Windsor Castle last June," a statement read.
"He also conveyed the great admiration of the American people for the Queen, whose dignity and constancy deepened the enduring friendship and special relationship between the United States and the United Kingdom."
There were emotional scenes on Tuesday evening when the late Queen was brought back to London by plane and taken to Buckingham Palace for the final time.
As the hearse carrying the coffin began its journey from Northolt in west London, people stood silently by the road with some recording the historic moment on their camera phones.
"She's given so much, I just wanted to give her a little something back", ITV News' Geraint Vincent speaks to mourners in London.
On Wednesday, the presence of King Charles, the Prince of Wales and the Duke of Sussex in the procession sees the father and sons united in their grief for a mother and grandmother.
The last time King Charles and his two sons were seen together in public was at the service of thanksgiving for the Queen in St Paul’s Cathedral during the June Platinum Jubilee celebrations.
But on that occasion, Prince Harry and Duchess Meghan were seated some distance from the King and Prince William on the other side of the aisle in the second row, behind the Wessex family and the Duke and Duchess of Gloucester.
The Jubilee service at St Paul’s was the Duke and Duchess of Sussex's first public appearance alongside the Windsors since they stepped down as senior royals in 2020.
Prince William and Prince Harry have had a well-documented fraught relationship in recent years but the brothers put on a united front with their wives during a surprise mammoth walkabout on Saturday.
They arrived at Windsor Castle in the same vehicle along with Princess of Wales, Kate and Duchess Meghan, before viewing floral tributes and greeting well-wishers for around 40 minutes before the Prince of Wales hopped into the driver’s seat with his wife in the passenger seat, and his brother and sister-in-law in the back.
In his televised address to the nation on Friday evening, the King talked of his love for his youngest son and daughter-in-law, saying: “I want also to express my love for Harry and Meghan as they continue to build their lives overseas.”
In the Duke of Sussex’s subsequent tribute to his grandmother, he said he wanted to honour his father at the start of his reign as King.
The remarkable life of the Queen remembered and the King's inaugural speech analysed in our latest episodes of What You Need To Know
In April 2021, Prince Harry and Prince William joined their father when they walked behind the Duke of Edinburgh’s coffin at his funeral.
The brothers were separated by their cousin Peter Phillips but he dropped back half a pace at one point so the siblings appeared closer together.
After the funeral service, the brothers could be seen chatting as they walked back up the hill from the chapel to the castle.