Thousands of mourners waited for hours in a line that stretched for nearly 7km across London for the chance to spend a few minutes filing past Queen Elizabeth II's coffin while she lies in state at Britain's Parliament.

King Charles III spent the day in private to reflect on his first week on the throne.

The queue to pay respects to the late Queen at Westminster Hall was at least a nine-hour wait, snaking across a bridge and along the south bank of the River Thames beyond Tower Bridge. But people said they didn't mind the wait, and authorities arranged portable toilets and other facilities to make the slog bearable.

"I'm glad there was a queue, because that gave us time to see what was ahead of us, prepared us and absorbed the whole atmosphere," health care professional Nimisha Maroo said. "I wouldn't have liked it if I'd had to just rush through."

Members of the public pay their respects as they walk past the coffin of Queen Elizabeth II, draped in the Royal Standard. Photo / Yui Mok, Pool via AP

A week after the Queen died at Balmoral Castle in Scotland after 70 years on the throne, the focus of commemorations was in Westminster — the heart of political power in London. Her coffin will lie in state at Westminster Hall until Monday, when it will be taken across the street to Westminster Abbey for the funeral.

Buckingham Palace on Thursday released details about the service, the first state funeral held in Britain since the death of former Prime Minister Winston Churchill in 1965. Royalty and heads of state from around the world are expected to be among the 2000 people attending, with a smaller burial service planned for later Monday at Windsor Castle.

Late Monday, the Queen will be buried in a private family service at Windsor alongside her late husband, Prince Philip, who died last year.

The guest list for the state funeral is a roll-call of global power and pomp, from Japan's Emperor Naruhito and King Felipe VI of Spain to US President Joe Biden, French President Emmanuel Macron and the prime ministers of New Zealand, Canada, and Australia.

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Canadian Prime Minister Trudeau — who first met the Queen when he was a child and his father Pierre Trudeau was Canada's leader — said the Queen was "one of my favorite people in the world".

People arrive in the queue at Westminster Palace to pay their respect to the late Queen Elizabeth II during the Lying-in State. Photo / Markus Schreiber, AP

"Her conversations with me were always candid, we talked about anything and everything, she gave her best advice on a range of issues, she was always curious, engaged and thoughtful," he said at a special session of the Canadian parliament in Ottawa.

After a day of high ceremony and high emotions on Wednesday as the Queen's coffin was carried in sombre procession from Buckingham Palace, the King was spending Thursday working and in "private reflection" at his Highgrove residence in western England. Charles has had calls with Biden and Macron and has been speaking to a host of world leaders.

Prince William, the heir to the throne, and his wife Catherine, the Princess of Wales, visited the royal family's Sandringham estate in eastern England on Thursday to admire some of the tributes left by well-wishers.

The couple walked slowly along metal barriers as they received bouquets from the public and chatted to well-wishers. Other royals fanned out across the UK to thank people for their support, with the Queen's youngest son, Prince Edward, and his wife Sophie visiting Manchester and the Queen's daughter Princess Anne in Glasgow.

People leave Westminster Palace after paying their respects, during the Lying-in State, at Westminster Hall in London. Photo / Markus Schreiber, AP

On Wednesday the Queen left Buckingham Palace for the last time, borne on a horse-drawn carriage and saluted by cannons and the tolling of Big Ben, in a solemn procession through the flag-draped, crowd-lined streets of London to Westminster Hall.

Charles, his siblings and sons marched behind the coffin, which was topped by a wreath of white roses and the Imperial State Crown on a purple velvet pillow. The military procession underscored Elizabeth's seven decades as head of state.

Her lying-in-state, meanwhile, allowed many Britons to say a personal goodbye to the only monarch most have ever known.

It's also a huge logistical operation, with a designated 16km queuing route lined with first aid points and more than 500 portable toilets. There are 1000 stewards and marshals working at any given time, and 30 religious leaders from a range of faiths to talk to those in line.

Monica Thorpe said she walked for two hours to get to the back of the line and join the queue.

"People were just walking and walking and the policemen were like 'Keep going, keep going.' It was like the yellow brick road," she said.

Archbishop of Canterbury Justin Welby, the spiritual leader of the Church of England, wore a high-visibility vest emblazoned with the words "Faith Team" as he spoke to mourners.

Thousands of flowers brought by the public outside Windsor Castle adorn the long walk where the Coffin of the late Queen Elizabeth II will be carried. Photo / Gregorio Borgia, AP

Welby, who will deliver a sermon at the Queen's funeral on Monday, paid tribute to the Queen as "someone you could trust totally, completely and absolutely, whose wisdom was remarkable".

People old and young, dressed in dark suits or jeans and sneakers, walked in a steady stream Thursday through the historic hall, where Guy Fawkes and Charles I were tried, where kings and queens hosted magnificent medieval banquets, and where previous monarchs have lain in state.

After passing the coffin, most mourners paused to look back before going out through the hall's great oak doors. Some were in tears; others bowed their heads or curtsied. One sank on to a knee and blew a farewell kiss.

Keith Smart, an engineer and British Army veteran, wiped away tears as he left the hall. He had waited more than 10 hours for the chance to say goodbye.

"Everybody in the crowd was impeccably behaved. There was no malice, everybody was friends. It was fantastic," he said. "And then, to come into that room and see that, I just broke down inside. I didn't bow — I knelt to the floor, on my knees, bowed my head to the Queen." 

— Jill Lawless & Mike Corder, AP