Members of the public are waiting in queues that have stretched to more than 25 hours.
Crowds greeted King Charles III today as he arrived at Buckingham Palace to meet Commonwealth leaders hours after a security scare interrupted the queen's sombre lying-in-state.
King Charles' arrival at the palace came as world leaders began to arrive in London ahead of Monday's state funeral, Britain's first in nearly six decades.
Princes William and Harry were later set to lead a vigil of Queen Elizabeth II's eight grandchildren at her coffin, joining tens of thousands of members of the public who have been queueing round the clock for days.
Those inside parliament's Westminster Hall for the lying-in-state received a shock late yesterday when a man burst out of the line and approached the coffin, which sits topped with the Imperial State Crown.
A live television feed of the mourners briefly cut away around 10:00 pm (2100 GMT) as police detained the man, two hours after Charles and his three siblings had held their own vigil in the cavernous hall.
"He was arrested for an offence under the Public Order Act and is currently in custody," London's Metropolitan Police said.
The queen's death on September 8 aged 96, after a record-breaking 70 years on the throne, has sparked an outpouring of emotion.
Members of the public are braving waits that have stretched to more than 25 hours, and chilly night-time temperatures, to view her flag-shrouded coffin.
Lines have snaked for miles along the River Thames since Wednesday when her coffin was brought to the UK parliament complex.
Some 435 people have needed medical treatment, often for head injuries after fainting in the queue, the London Ambulance Service said.
But Fiona Campbell-Bell, 46, was undeterred as she joined the line.
"I've got an iPad, I have downloaded quite a few things to watch, I've got a very thick winter coat and hopefully that will get me through," she told AFP.
Police are mounting Britain's biggest-ever security operation for Monday's funeral, as hundreds of dignitaries including US President Joe Biden are set to jet in.
Less than two weeks into her premiership, British Prime Minister Liz Truss was on Saturday beginning a packed weekend of meetings with world leaders including her counterparts from New Zealand, Jacinda Ardern, and Australia, Anthony Albanese, at the government's Chevening country residence.
On Sunday she was due to meet Biden, Irish Taoiseach Micheal Martin, Canadian premier Justin Trudeau and Polish leader Andrzej Duda in Downing Street.
King Charles, meanwhile, was on Saturday scheduled to meet the prime ministers of the Commonwealth realms -- the 14 former colonies over which he now reigns in addition to the United Kingdom -- including those of Australia, The Bahamas, Canada, Jamaica and New Zealand.
From Australia and Canada to Jamaica and Papua New Guinea, they have formally proclaimed him their new sovereign.
But republican movements are gaining ground in many of the countries, and efforts to keep them all in the royal fold will likely be a feature of his reign.
Charles on Friday wrapped up his maiden tour as monarch to the four UK nations with a visit to Wales, part of an operation dubbed "Spring Tide" to launch him in his new role.
'Tide of emotion'
Back in London, Charles joined the 15-minute vigil with his siblings -- Princess Anne, Prince Andrew and Prince Edward -- around their mother's casket on Friday night.
They stood, silent and eyes lowered, while members of the public filed past.
The vigil will be repeated on Saturday evening by the eight grandchildren, including the new heir to the throne Prince William and his estranged brother Harry.
Harry -- who served two tours with the British army in Afghanistan -- has been given special permission by his father to wear military uniform despite no longer being a working royal.
The move appeared to be the latest olive branch offered to Harry by Charles after the prince and his wife Meghan, now living in California, accused the royal family of racism.
The personal sorrow of the queen's family has been playing out in the glare of intense international attention.
But the queen's youngest son Edward said: "We have been overwhelmed by the tide of emotion that has engulfed us and the sheer number of people who have gone out of their way to express their own love, admiration and respect."
The public have until early Monday to view the coffin before the queen is honoured with Britain's first state funeral in nearly six decades.
The spectacular ceremony at Westminster Abbey -- expected to be watched by millions around the globe -- will see 142 sailors pulling the gun-carriage bearing her lead-lined coffin.
It will be attended by more than 2,000 guests, but leaders from countries at loggerheads with the UK such as Russia, Belarus and Afghanistan have not been invited.
China's vice president Wang Qishan will attend, Beijing's foreign ministry confirmed, after a diplomatic spat saw Chinese officials barred from visiting the coffin inside parliament.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)