Protesters are targeting Trooping the Colour as a "substantial" police operation has been put in place.
Hundreds of Metropolitan Police officers have been deployed along The Mall to ensure the safety and security of attendees, as the anti-monarchy group Republic has arrived to protest King Charles and the Royal Family.
Scotland Yard has a "substantial" policing operation for the event, despite the anti-monarchy group Republic being allowed to protest.
However, they have been banned from using amplified sound to distract from the Trooping the Colour parade.
Kate and Prince William will be attending Trooping the Colour today
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In a post on social media today, Republic said: "It’s a lovely sunny day (so far) for a protest! Join us, outside Buckingham Palace from 9.00am. #NotMyKing #AbolishTheMonarchy."
Tens of thousands of royal fans are expected to arrive at The Mall today to witness Princess Kate, 42, make her first public appearance this year.
Kate, who has been receiving treatment for an undisclosed form of cancer, will ride in a carriage through London with her three children, Prince George, Princess Charlotte and Prince Louis.
Royal fans have been camping outside Buckingham Palace in the hopes of catching a glimpse of the Princess of Wales today.
Republic has gathered at The Mall to protest Trooping the Colour
PA
Although the traditional ceremony officially begins at 10am, they have been camping outside since early this morning.
The Princess of Wales will join other royals on Buckingham Palace's balcony later today, and staff have been busy getting it ready this morning.
The Trooping the Colour ceremony takes place to celebrate the birthday of the reigning monarch in the UK.
It has been held every year for the past 260 years.
Republic protesters are renowned for shouting and holding placards reading 'Not My King'
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Metropolitan police have gathered to protect the safety of attendees at Trooping the Colour
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The popular event will attract big crowds, who will wave flags as the parade moves from Buckingham Palace and down The Mall to the Horse Guard's Parade, alongside Members of the Royal Family on horseback and in carriages.
During the ceremony, King Charles will be greeted by a royal salute, before carrying out an inspection of the troops.
The troops are fully trained operational soldiers, who wear the ceremonial uniform of red tunics and bearskin hats.
The ceremony will begin at 10am and end around 1pm, with members of the Royal Family appearing on the Buckingham Palace balcony to conclude the festivities.
