LONDON - King Charles was due on Saturday to meet leaders from his new Commonwealth realms ahead of his mother's ornate funeral, after a security scare interrupted her sombre lying-in-state.

Princes William and Harry were later set to lead a vigil for Queen Elizabeth's eight grandchildren at her coffin, joining tens of thousands of members of the public who have been queuing round the clock for days.

Those inside Parliament's Westminster Hall for the lying-in-state received a shock late on Friday 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 as the police detained the man, two hours after King Charles and his three siblings 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 Sept 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 British Parliament complex.

Some 435 people have needed medical treatment, often for head injuries after fainting in the queue, the London Ambulance Service said.

But Ms Fiona Campbell-Bell, 46, was undeterred as she joined the queue.

"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 the night," she said.

The police are mounting Britain's biggest-ever security operation for Monday's funeral, as hundreds of dignitaries, including United States President Joe Biden, are set to jet in.

King Charles will meet on Saturday with the prime ministers of the Commonwealth realms - the 14 former colonies over which he now reigns, in addition to Britain.

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. AFP