Five soldiers collapsed outside St Paul's Cathedral ahead of a service of thanksgiving to celebrate the Queen's Platinum Jubilee.
It happened just before the royals arrived at the service this morning.
The first incident saw a Royal Air Force soldier falling to the ground. He was soon helped back onto his feet by colleagues and was led away to recover.
The second soldier is understood to be a member of the Canadian artillery, although he continued to stand guard on the steps of St Paul's as guests arrived.
Three further soldiers are also said to have collapsed, according to reports.
Video footage showed politicians arriving at the cathedral for the service with huge crowds gathered outside.
It appeared that the first soldier had collapsed just before Liz Truss, the foreign secretary, walked up the steps and into St Paul's.
Crowds that had gathered to watch the famous faces arrive for the service cheered when the solider was helped to his feet.
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On Twitter, the associate editor of the Telegraph tweeted: "A soldier has just keeled over, received medical attention and is now back on his feet with a cheer."
The solider was then walked away by colleagues ahead of the service.
It happened while a number of people were arriving for the thanksgiving service, including Boris Johnson and his wife Carrie.
It appeared the prime minister was booed and whistled by the crowds, while others cheered, as they climbed up the steps.
The royals have also arrived at the event, including Prince Harry and Meghan Markle, although the Queen will be sitting out the service.
The monarch, 96, was due to attend St Paul's but pulled out last night after feeling "some discomfort" during yesterday's Trooping the Colour and the RAF flypast.
A statement issued by the Palace on Thursday evening said: "The Queen greatly enjoyed today's birthday parade and flypast but did experience some discomfort.
"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."
Meanwhile Prince Andrew tested positive for Covid-19 and will also be missing the service, it was confirmed.
Ahead of the service at St Paul's, the Duke and Duchess of Sussex arrived, with the crowds heard cheering as they entered the cathedral.
Meghan was wearing a cream dress and matching hat, and arrived shortly after 11am.
One crowd member, however, could be heard shouting at the pair: “I don’t know how you’ve got the nerve to show up.”
The pair were seen holding hands as they were escorted to their seats next to other members of the royal family.
Just minutes later, the Duke and Duchess of Cambridge arrived but were without Prince George, Prince Louis and Princess Charlotte this time.
Prince William and Kate, who was wearing a warm, yellow dress, were met with loud cheers from the crowd.
Prince Charles and Camilla have also arrived for the special thanksgiving service.
A number of former prime ministers are in attendance as well, including Tony Blair, Gordon Brown, David Cameron and Theresa May.
The Archbishop of Canterbury had to pull out of the service after testing positive for Covid-19 last week.
Justin Welby, the Church of England's most senior cleric, was due to deliver a sermon. Stephen Cottrell, the Archbishop of York, is speaking instead.
The Royals join in for the first hymn at the service with Dr Ison leading the congregation in a prayer, with the State Trumpeters of the Household Cavalry playing to the church.
Other speakers included Boris Johnson who have a reading from the New Testament.
More than 400 people, including key workers, armed forces members and and charity volunteers are at the service in recognition for their public life.
