As mourners streamed 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 his youngest son after Harry and his wife Meghan, now living in California, accused the royal family of racism.
No longer a working royal, Harry, 38, was stripped of his higher-ranking honorary military titles. The vigil will be the only time he will be seen in military dress at royal ceremonial occasions.
The king and his eldest son William, the new heir to the throne, had earlier surprised those standing in line along the Thames by staging an unscheduled walkabout to shake their hands and thank them for coming.
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 in London at 11:00 am (1000 GMT).