London CNN  — 

Britain’s Queen Elizabeth II will miss a service of thanksgiving at St Paul’s Cathedral for her 70 year reign on Friday, after experiencing “some discomfort” during the first day of her Platinum Jubilee celebration.

The Queen appeared at Buckingham Palace on Thursday during the festivities and participated in a beacon lighting ceremony from Windsor Castle in the evening.

“The Queen greatly enjoyed today’s Birthday Parade and Flypast but did experience some discomfort,” Buckingham Palace said in a statement.

“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,” it continued.

Earlier Thursday, the 96-year-old Queen looked on from the palace’s famous balcony at a sea of red, white and blue as huge, flag-waving crowds flocked to the Mall to see the monarch and her family.

The area around Buckingham Palace and nearby St James’ Park was packed with homegrown and international tourists – some of whom had camped out overnight – to witness the first event of the bumper four-day weekend celebrations.

And after seven decades of duty, the sovereign certainly lived up to her reputation as a loyal crowd-pleaser. She may not have been up to the ceremonial role of riding on horseback as part of Trooping the Colour, but she nonetheless delighted fans by her presence on the balcony following the military parade.

Dressed in a dusky dove blue Angela Kelly ensemble with matching hat, the Queen smiled as she received the salute as soldiers and officers returned from Horse Guards Parade following the conclusion of her birthday parade.