Anti-monarchy group Republic said it would hold the largest protest in its 50-year history on the day of King Charles' coronation.
Speaking to The Times, the group's head Graham Smith said more than 1,350 people have pledged to protest on the day of the coronation.
He said the group's members will wear yellow T-shirts and wave placards to create an "unmissable sea of yellow" along the procession route in central London.
They plan to boo the new monarch as he passes in his gold stage coach while chanting: "Not my king".
Most of the demonstration will be in Trafalgar Square but smaller groups of anti-monarchists will be dotted along other sections of the route.
He said the demonstrators are expected to arrive early in the morning to be as close to the barriers as possible.
The activist said that they were not planning any Extinction Rebellion-style stunts, because "it’s not a good look" and "doesn’t help the cause".
Smith added, "We want to make sure we have as many people as possible when Charles goes past, which we assume will be between 10 and 10:30am. You hope to be reasonably down near the front with a sea of placards.