London CNN  — 

Mourners queued for miles on Wednesday to say goodbye to Queen Elizabeth II as her coffin began lying in state, after senior royals including King Charles III, Prince William and Prince Harry accompanied it on a procession from Buckingham Palace to Westminster.

When Westminster Hall opened its doors at 5 p.m., the queue of visitors stretched for almost three miles along the River Thames, according to an official tracker.

The coffin will remain at the hall, in the heart of the British parliamentary estate, until the morning of the Queen’s funeral on Monday.

Senior royals walked behind the coffin as it made its way along the procession route, carried on a gun carriage.

Prince William and Prince Harry also marched side by side in a funeral procession for their mother, Princess Diana, in 1997, a moment which both have said affected them deeply.

The Queen's coffin lying in state in Westminster Hall.

The coffin was adorned with the Imperial State Crown next to a wreath of flowers, and covered with the Royal Standard.

It was surrounded by military personnel as it was carried along The Mall, Whitehall, Parliament Street, Parliament Square and New Palace Yard, before entering Westminster Hall, where a short service was conducted by the Archbishop of Canterbury Justin Welby.

Prince William and Prince Harry follow the coffin of Queen Elizabeth II.

Welby told CNN that it is a “gift” to “play a part in saying goodbye” to Elizabeth.

Speaking to CNN’s Clarissa Ward on Wednesday in the crowd, the Archbishop said that it is a huge privilege and honor to be part of the occasion.

King Charles pictured during the procession.

The Queen’s coffin was constructed more than 30 years ago, and is made from English oak lined with lead, the royal undertakers Leverton & Sons told the British newspaper The Times.

From Wednesday evening, members of the public were able file past the coffin, which rests on a raised platform or catafalque. Thousands are expected to queue along the streets of the capital, with some potentially sleeping out overnight in a bid to pay their respects.

Harry’s inclusion in Wednesday’s procession is the latest in a series of appearances with William since the Queen’s death, as the brothers have sought to put on a show of unity despite tensions in recent years since Harry quit as a senior royal.

A joint appearance of William and Harry and their wives Catherine, Princess of Wales and Meghan, Duchess of Sussex in Windsor on Saturday came as a surprise and was not announced in advance. William invited the Sussexes to the walkabout to see memorials left for the Queen and greet well-wishers, a source told CNN.

The coffin, covered with the Royal Standard and adorned with the Imperial State Crown.

The Queen’s coffin was transported from Balmoral Castle, where she died, to Edinburgh on Sunday. The six-hour route marked the first leg of the late monarch’s eight-day journey to her final resting place.

Mourners queued outside St. Giles’ Cathedral in Edinburgh on Monday evening for their turn to pay their final respects. The Scottish government said more than 26,000 people got to file past the Queen.

On Tuesday, the Queen’s coffin was transported from St. Giles’ Cathedral in Edinburgh to Buckingham Palace, London.

It was taken from the cathedral to Edinburgh airport in a hearse before being flown to RAF Northolt airbase in west London on board a C-17 Globemaster transport plane.

The coffin was then driven to Buckingham Palace, where it was received by the Queen’s family and then left to rest in the Bow Room overnight.

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