LONDON: Britain's royal family and Prime Minister Boris Johnson attended a service of thanksgiving for Queen Elizabeth on Friday, the second day of her Platinum Jubilee celebrations, although the monarch herself was missing due to ongoing mobility issues.

Four days of events kicked off on Thursday, when a beaming Elizabeth waved to crowds from the balcony of Buckingham Palace after a military parade and Royal Air Force flypast, and later led the lighting of the Principal Platinum Jubilee Beacon at her Windsor Castle home.

The celebrations continued with a National Service of Thanksgiving at London's St Paul's Cathedral to pay tribute to the sovereign's 70 years on the throne.

Queen Elizabeth II’s 70 years on the throne: key dates

But the queen, who is 96 and has been forced to cancel a series of engagements recently due to "episodic mobility problems", reluctantly pulled out of Friday's service.

Buckingham Palace said she had experienced "some discomfort" during Thursday's events, and the journey to London from Windsor Castle, where she spends most of her time these days, and the activity involved for the service was too much.

Her son and heir Prince Charles, 73, instead represented Elizabeth, who is a devout Christian and also the titular head of the Church of England. She will watch the service on television at her home in Windsor Castle near London.

"We are all disappointed, she has said how disappointed she is but we want her to be well," Britain’s chief rabbi, Ephraim Mirvis, told BBC TV outside St Paul's.

Queen Elizabeth to miss Friday Jubilee event due to mobility discomfort

"I think she's been magnificent so far over this week, and she should look after herself, she should pace herself, and it's fine: please God, she should live many, many more years in good health."

Also absent from the service was her second son, Prince Andrew, 62, who has tested positive for COVID-19. read more

That potentially spares the royals some awkwardness, with Andrew's reputation shattered after he settled a U.S. lawsuit in February in which he had been accused of sexually abusing a woman when she was underage, claims he denied.

However, the queen's grandson Prince Harry and his wife Meghan were there, making their first public appearance together in Britain since stepping down from royal duties two years ago. They were greeted by cheers and a few boos on their arrival.

The couple moved to the United States to lead a more independent life, and have since delivered some stinging attacks on Buckingham Palace and the royal family.

The service features Bible readings, prayers, and hymns to express gratitude for Elizabeth's reign. Among the congregation were political figures from Britain and across the world.

Prime Minister Boris Johnson, who will give a reading, was met by a mixture of cheers and boos from the crowd outside the cathedral, reflecting recent public anger over his conduct in office. read more

"We come together in this cathedral church today to offer to God our thanks and praise for the reign of Her Majesty the Queen and especially for her 70 years of faithful and dedicated service," David Ison, the Dean of St Paul's, will say.

The cathedral's 'Great Paul' bell - the largest in the country and dating back to 1882 - will also be rung for the first time at a royal occasion since being restored last year after a mechanism broke in the 1970s.

After the service, a reception will be held at the Guildhall hosted by the Lord Mayor of the City of London.

Thursday marked not only the start of the Jubilee, but also the 69th anniversary of the coronation of Elizabeth, who became queen on the death of her father George VI in February 1952 and is head of state of 14 other countries including Australia, Canada and New Zealand.

She has now been on the throne for longer than any of her predecessors in 1,000 years, and is the third-longest reigning monarch ever of a sovereign state. Opinion polls show she remains hugely popular and respected among British people.

Platinum Jubilee celebrations: Beaming Queen Elizabeth waves to crowds

Roxie Kishore-Bigord, 51, who was outside St Paul's to see the guests arriving, said it was disappointing the queen had been unable to attend.

"We want her well, we want her to keep going," she told Reuters. "We're happy that she will probably be watching from home and hopefully she will see how much she's loved and appreciated."