Queen Elizabeth lit the Principal Beacon at the official dual lighting ceremony on Thursday after news broke that she will not be attending the Service of Thanksgiving on Friday.
The Queen made her arrival as her grandson, Prince William and others looked on. William appeared moved during the moment.
The Queen touched the Commonwealth Nations Globe to start the lighting ceremony, with beacon pageant master Bruno Peek, who has overseen the Platinum Jubilee Beacons Project from its inception.
William was greeted by the Chair of The Queen's Green Canopy, Sir Nicholas Bacon, and the designer of the "Tree of Trees," Thomas Heatherwick.
INSIDE TROOPING THE COLOUR: AN EMOTIONAL QUEEN ELIZABETH TO A NERVOUS PRINCE WILLIAM
Queen Elizabeth attended the Trooping of Colour ceremony earlier in the day before the palace shared a statement that she will not be attending Friday's festivities due to "some discomfort."
"The Queen greatly enjoyed today’s Birthday Parade and Flypast but did experience some discomfort," a statement from Buckingham Palace said. "Taking into account the journey and activity required to participate in tomorrow’s National Service of Thanksgiving at St Paul’s Cathedral, Her Majesty with great reluctance has concluded that she will not attend."
QUEEN ELIZABETH THROUGH THE YEARS
"The Queen is looking forward to participating in tonight’s Beacon lighting event at Windsor Castle and would like to thank all those who made today such a memorable occasion," the statement concluded.
The Queen is marking 70 years on the throne became the first British monarch to see a Platinum Jubilee. The festivities began on June 2 and will conclude on June 5.
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Buckingham Palace previously shared that the queen’s attendance at events throughout the Platinum Jubilee weekend will likely only be confirmed on "the day itself."
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Queen Elizabeth has suffered from "episodic mobility problems" this past year and has limited her social appearances. Her son, Prince Charles, has stepped in on her behalf on several occasions.
Fox News' Tracy Wright contributed to this report.