King Charles III

US President Joe Biden was to pay his last respects in London to Queen Elizabeth II on Sunday, as ordinary mourners waiting in marathon lines were warned that time was running out to view her coffin lying in state.

After witnessing the sombre scene in parliament's Westminster Hall, Biden, Japan's Emperor Naruhito and other world leaders were due to attend a reception with King Charles III.

Biden, who flew in late Saturday, has said that Charles's mother "defined an era" after she reigned for a record-breaking 70 years leading up to her death on September 8, aged 96.

Australia's anti-monarchy Prime Minister Anthony Albanese, who viewed the lying-in-state and met Charles on Saturday, told Sky News Australia that the Queen was "a constant reassuring presence".

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There was also a private audience at Buckingham Palace for Prime Minister Jacinda Ardern of New Zealand, which like Australia and 12 other Commonwealth realms now counts Charles as its sovereign.

"You could see that it meant a huge amount (to Charles) to have seen the sheer scale and outpouring of people's love and affection for her late Majesty," she told BBC television Sunday.

US President Joe Biden signs sign a book of condolence at Lancaster House in London on Sunday. AFP

But in a sign of challenges ahead for the new king, Ardern added that she expected New Zealand to ditch the UK monarchy "over the course of my lifetime".

Members of the public were already camping out in advance to catch a glimpse of Monday's grand farewell at Westminster Abbey, which is expected to bring London to a standstill and be watched by billions of viewers worldwide.

Country's 'glue'

E.J. Kelly, a 46-year-old school teacher from Northern Ireland, secured a prime spot with friends on the route the procession will take after the funeral.

"Watching it on television is wonderful but being here is something else," she told AFP, equipped with camping chairs, warm clothing and extra socks.

"I will probably feel very emotional when it comes to it, but I wanted to be here to pay my respects."

Crowds also thronged around Windsor Castle, west of London, where the Queen's coffin will be driven after the service for a private burial to lay her to rest alongside her late husband Prince Philip, her parents and her sister.

Britain's King Charles III (left) walks behind the coffin of Queen Elizabeth II in London on Wednesday. AFP

"I've lived here my whole life and I've never seen it this busy," said Donna Lumbard, 32, a manager at a local restaurant.

Starting with a single toll from Big Ben, British Prime Minister Liz Truss will lead a national minute's silence at 8:00 pm (1900 GMT) on Sunday to reflect on the "life and legacy" of the Queen.

Near the Scottish town of Falkirk, 96 lanterns were to be lowered into a "pool of reflection" at the foot of the Queen Elizabeth II Canal, before wreaths are placed in the water.

A balloon with a picture of Queen Elizabeth II is photographed at the Green Park memorial in London. AP

Those wanting to view the flag-draped casket have until 6:30 am (0530 GMT) on Monday to make it into the cavernous Westminster Hall opposite the abbey.

As the queue continued to snake for miles (kilometres) along the River Thames on Sunday, the waiting time stood at more than nine hours, and the line is likely to be closed by the evening.

"To avoid disappointment please do not set off to join the queue," the government said.

Andy Sanderson, 46, a supermarket area manager, was in the line and finally reaching parliament.

"She was the glue that kept the country together," he said.

"She doesn't have an agenda whereas politicians do, so she can speak for the people."

Agence France-Presse