US President Joe Biden paid his last respects in London to Queen Elizabeth II on Sunday, as time ticked down for ordinary mourners to view her coffin ahead of the funeral.

Biden crossed himself and put his hand on his heart as he stood with his wife Jill on a gallery overlooking the flag-draped casket in the cavernous Westminster Hall while members of the public filed by.

After witnessing the sombre scene, the US leader, Japan's Emperor Naruhito, French President Emmanuel Macron and other heads of state from around the world headed for a reception with King Charles III.

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

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".

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.

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.

"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.

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

- Grandchildren's vigil -
 

As mourners slowly filed 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 Harry and his wife Meghan after they quit royal duties and moved to North America, later accusing the royal family of racism.

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 at 11:00 am.

Reflecting on the queen's wishes for the hour-long ceremony, the former archbishop of York, John Sentamu, said she "did not want what you call long, boring services".

"The hearts and people's cockles will be warmed," he told BBC television.

- Tributes from Camilla, Andrew -
 

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.

As their private grief has played out in the glare of global attention, a fresh opinion poll from YouGov showed the royal family's popularity has risen in the UK.

William and his wife Kate topped the ranking of most popular royals while Charles saw his approval ratings rise 16 points since May.

The queen's second son Prince Andrew, in disgrace over his links to billionaire US paedophile Jeffrey Epstein, paid tribute Sunday to the queen's "knowledge and wisdom infinite, with no boundary or containment".

Camilla gave her first public comments as the new queen consort, recalling 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, Charles's wife 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."