LONDON — Britain is headed for its biggest security challenge since World War II as more than 70 heads of government and hundreds of other dignitaries arrive for Queen Elizabeth II’s funeral and some 2 million people are expected to take to the streets. With the royal family planning to walk behind the Queen’s coffin, former London correspondents Mary Jordan and Kevin Sullivan write, “police are trying to strike a balance between security and worship”. The security question looms large, and not just because a man was arrested late Friday after loading into the Queen’s coffin. On the ground: People pour into the city, snarling traffic and crowding the sidewalks late at night. The queue to pay respects to the Queen is more than five miles, winding from Westminster to a 60-acre park in south-east London. Officials closed the line, affectionately dubbed ‘The Queue’, for a few hours overnight when waits were expected to reach 25 hours. As of late Saturday afternoon, the official tracker expects about 13 hours. “I have been to the stadium many times. This was different” London correspondent William Booth writes of his experience at 2am seeing the Queen’s coffin lying in state. People queuing along the Thames on Saturday welcomed royal visitors: King Charles III and his eldest son William. “He would never believe that, honestly. It’s amazing,” William told some in the crowd, Sky News reports. Sartorial soap opera: The Queen’s four children – King Charles, Princess Anne, Prince Andrew and Prince Edward – stood a silent vigil around her coffin on Friday afternoon in Westminster Hall as other members of the royal family looked on. The sight of so many elderly kings parading in uniform is for some a sight worth questioning. (See: Anne’s hat, below.) What we know: The sovereign heads the UK’s armed forces, and senior members of the royal family hold honorary positions with different regiments. A number of royals have served in the military, including Princes William and Harry, who wore their Blues and Royals uniforms when they led many of their cousins to their grandchildren’s vigil on Saturday night. But people who question the relevance of the monarchy today are likely to see it as an odd moment of royal attire on an otherwise solemn occasion. POTUS Notice: President Biden and first lady Jill Biden are expected to arrive here on Sunday. Their schedule includes signing an official book of condolence for the Queen and attending a diplomatic reception hosted by King Charles. On Monday, they attend the queen’s funeral and then return to the United States. (ICYMI: Opinions columnist James Hohmann thinks the president can teach the new king a few things.) Royal Composer: The Post’s classical music critic Michael Andor Brodeur writes a Q&A with Judith Weir, Master of the King’s Music, a 396-year-old post dating back to the first King Charles (“back when music was music”). These days the role is akin to that of a poet laureate – “a link between the wide world of music and the relative emptiness of the palace,” he writes. Weir, the first woman to hold the position of teacher, says the late Queen had “immense respect for musicians”. Religious Symbolism: Although the queen may be gone, Karen Attiah writes in her Post Opinions newsletter, “the legacy of white Christian supremacy lives on.” Graphic: Britain’s royal line of succession, visualized Coverage from around The Post Where will the queen be buried? This interactive by Aaron Steckelberg, Manuel Canales and Ruby Mellen takes a look at Elizabeth II’s final resting place. Two dozen members of the royal family are buried in a vault beneath St George’s Chapel in Windsor. On the democracy issue, Australia has been here before, Correspondent Michael E. Miller writes from Windsor (yup, a town with the same name as the royals). A 1999 democratic referendum — which could have removed the monarch as head of state — revealed deep divisions: “Many urban areas voted for the proposed democracy, while more conservative places like Windsor rejected it. What killed the initiative, however, was that Republicans could not agree on how to choose an Australian head of state.” There are still few signs of consensus, Miller writes. Big picture: Colonial history and the meaning of independence are debated from the Caribbean to the Pacific. Australian Prime Minister Anthony Albanese hopes to hold a new referendum on the issue within six years. “Charles could be an eco-warrior king in a Savile Row suit” write London correspondents William Booth and Karla Adam, looking at how the new king may be different from his mum. While the Queen has been hard to track down, Charles is a crusader at heart. “Having just been dismissed by his critics for confessing to talking to trees, Charles is in time for 2022.” The bow that broke the anti-monarchy fever. In the days following Princess Diana’s death in 1997, writes Gillian Brockle in Retropolis, public outrage grew at the absence of visible royal mourning. “People who hadn’t thought it through were openly questioning whether Britain needed a monarch at all.” These gestures from the Queen signaled that Elizabeth was changing course — and gave Diana her due. Queen Elizabeth’s life in pictures. Our gallery charts her evolution from a princess born in 1926 to Britain’s longest-serving monarch. The Prince and Princess of Wales (@princeandprincessofwales) met on Friday with troops from Commonwealth countries who will take part in funeral ceremonies. Reports this week suggest the couple will visit Australia next year. 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