Queen's children hold vigil at Westminster Hall over her coffin.

King Charles led a second vigil with his siblings as the crowd passed.

Queen Elizabeth II's four children, headed by the King, quietly marched down the stone steps within Westminster Hall and stopped in front of the catafalque, where the vigil of the princes had begun.

It only took three taps until they walked on it. With his eyes red and squinting, King Charles stood at the head of the coffin, with Princess Royal and the Earl of Wessex on either side and the Duke of York at the foot.

One by one, they started facing outside. Then they turned their backs on the casket and stood there, heads lowered, unmoving.

A state funeral will be held on Monday, but today was the family's last chance to pay their respects to their mother and Queen in a moving ceremony. And just like so many other times this past week, a private moment was broadcast for all to see.

As the King and his brothers and sisters stood guard, the crowd filed slowly past them on both sides of the big hall. Queen Consort and Countess of Wessex also attended and observed the occasion. After 15 minutes, the four walked down, and with another triple tap, their mission was accomplished.

Her children had already done the same at St. Giles' Cathedral in Edinburgh, where she was put to rest before her body was returned to Buckingham Palace. Prince Andrew, who typically sticks to morning dress due to his nonworking royal position, wore a military uniform to Westminster Hall.

It has been revealed that the Queen's eight grandchildren will keep vigil over her casket on Saturday night. To begin, the Prince of Wales will take the helm, followed by the Duke of Sussex in the very last row. Harry, a nonworking member of the royal family, will be wearing his uniform for this momentous occasion with the King's blessing. The rest of the grandchildren will be wearing morning jackets and formal, dark attire with festive trimmings.

First observed in 1936, the vigil of the princes was held by King George V's four sons as they stood guard over their father's casket. In 2002, Queen Elizabeth II's four grandsons stood to watch over their grandmother's casket in Westminster Hall.