Charles is set to face chants of 'Not my King' on the day of his Coronation as an anti-monarchy group stages the largest protest action in its history.
Graham Smith, Republic's chief executive, said activists will wear yellow T-shirts and carry yellow placards in groups along the procession route, and also gather for a major demonstration in Trafalgar Square on May 6.
He has vowed to make the peaceful demonstration unmissable and loud, but said they were not staging any Extinction Rebellion-style stunts because "it's not a good look" and "doesn't help the cause".
More than 1,350 people had pledged to take part so far, he added.
His comments come as a new poll suggests that more than half of Britons do not think the King’s Coronation should be funded by the Government.
Mr Smith told The Times: "Anti-monarchy protests will carry one simple message: Do you want Charles or do you want a choice?"
He also branded the crowning of Charles and the Queen Consort a "pointless piece of theatre" which will cost tens of millions of pounds and be a "slap in the face" for people struggling with the cost-of-living crisis.
Protesters have already targeted Charles at recent public appearances, with their "Not my King" signs and shouts of "Why are you wasting money on a Coronation Charles?".
The group is asking people to sign a pledge to protest and to add their voices to their call for the UK to become a republic and have an elected head of state.
"On Saturday 6th May the eyes of the world will be on the coronation," its website says. "This is the moment we make our objection loud, visible and impossible to ignore."
The campaigners have written to every police force in the UK including the Metropolitan Police asking for reassurances they will not interfere in "peaceful and meaningful" protests against the monarchy.
Meanwhile, a YouGov survey - carried out just over two weeks before Charles and the Queen Consort are set to be crowned - has found that 51% of adults questioned believe the ceremony should not be funded by the Government.
Almost a third – 32% – said it should, while around 18% did not know.
The King’s coronation is set to cost many millions of pounds - and it falls to taxpayers to foot the bill.
But with no budget revealed for the historic national state occasion, and the Government not commenting on the expected total cost, the amount of public funds due to be spent remains unknown.
Taking place amid the cost-of-living crisis facing the UK and against a backdrop of strikes by doctors, teachers and other public servants over pay, the event has been branded a waste of taxpayers’ money by critics.
Some unconfirmed predictions suggest Operation Golden Orb could cost the nation between £50 million and £100 million.
The late Elizabeth II’s coronation cost £912,000 in 1953 – £20.5 million in today’s money – while Charles’s grandfather George VI was crowned at a cost of £454,000 in 1937 – worth £24.8 million in 2023 and the most expensive coronation of the last 300 years.
Of the 4,246 adults surveyed on Tuesday, 62% of those aged 18 to 24 were not in favour of the coronation being Government-funded, while 15% were in favour.
With those aged 65 and over, the figures were more evenly balanced, with 44% saying it should not be Government-funded, and 43% saying it should.
For 25 to 49-year-olds, 25% said the coronation should be Government-funded and 55% said it should not, and for 50 to 64-year-olds 46% said it should not be down to Government money, while 39% said it should.
the King and the Government are “mindful of ensuring that there is value for the taxpayer” and there will not be “lavishness or excess”.
But Mr Dowden also told the Commons Public Administration and Constitutional Affairs Committee at the start of the year: "It is a marvellous moment in our history and people would not want a dour scrimping and scraping."
