US President Joe Biden was Sunday to pay his last respects in London to Queen Elizabeth II as ordinary mourners queued for the final 24 hours left 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 the late queen's successor, King Charles III.

Biden, who flew in late Saturday, has said that Elizabeth, who was queen for a record-breaking 70 years, "defined an era".

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

Prime Minister Justin Trudeau of Canada, which like Australia now has Charles as its sovereign, said she "served for her entire life, and bore the weight of her duties with impeccable grace".

They are among dozens of heads of state and government in attendance, as Britain stages its biggest-ever policing operation for the historic funeral of its longest-reigning monarch.

The first 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.

- 'The nation's granny' -

"We wanted to have a good spot to see the procession," said Royal Navy veteran Bill Parry, 59, as he waited in camping chairs with two other former servicemen.

"It's not much to ask to sleep outside considering everything the queen did for us: 70 years of duty."

Britain will hold a minute's silence at 8:00 pm (1900 GMT) on Sunday to reflect on the "life and legacy" of the queen ahead of her funeral.

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 Sunday morning, the waiting time stood at more than 13 hours, and the line could be closed early at some point.

IT worker Shaun Mayo, 27, was among those relieved to have made it to Westminster Hall after queueing for 14 hours to pay his respects.

"It was incredibly emotional. She was like the nation's granny," he told AFP.

"We'll all miss her."

A man who was arrested after approaching the casket on Friday has been charged with a public order offence, police said.

- Grandchildren's vigil -

As mourners streamed by on Saturday evening, Prince William and his estranged younger brother Prince Harry led the queen's eight grandchildren in a 12-minute vigil around the coffin.

Harry -- who did two tours with the British Army in Afghanistan -- wore the uniform of the Blues and Royals cavalry regiment in which he served.

The move appeared to be the latest olive branch offered by Charles towards his youngest son after Harry and his wife Meghan, now living in California, accused the royal family of racism.

No longer a working royal, Harry, 38, was stripped of his higher-ranking honorary military titles. The vigil will be the only time he will be seen in military dress at royal ceremonial occasions.

The king and his eldest son William, the new heir to the throne, had earlier surprised those standing in line along the Thames by staging an unscheduled walkabout to shake their hands and thank them for coming.

Queen Elizabeth's state funeral, the first in Britain since the death of her first prime minister Winston Churchill in 1965, will take place Monday at Westminster Abbey in London at 11:00 am (1000 GMT).

- Russia says 'immoral' -

Leaders from Russia, Afghanistan, Myanmar, Syria and North Korea were not invited to join the 2,000 guests.

Moscow's foreign ministry last week called the decision "deeply immoral", and "blasphemous" to the queen's memory. China will attend at the abbey, but was barred by parliamentary leaders from the lying-in-state.

More than 2,000 police officers have been drafted from across the country to help Scotland Yard.

After the funeral, the queen's coffin will be transferred to Windsor Castle, west of London, for a private burial when she will be laid to rest alongside her late husband Prince Philip, her parents and her sister.

Camilla, the new queen consort, was the latest royal to pay tribute as she remembered her mother-in-law's smile and "wonderful blue eyes".

"It must have been so difficult for her being a solitary woman" in a world dominated by men, Camilla said in televised comments.

"There weren't women prime ministers or presidents. She was the only one so I think she carved her own role."