"I don’t believe in hereditarian monarchy and I think this is costing a huge amount of money,'' protester Colin Dear, a teacher in his forties told Reuters.
Republic, a group founded in 1983 that campaigns for an elected head of state, hope this will be their biggest protest ever. They believe Charles' accession to the throne presents their best chance of ending the monarchy, which traces its history back more than 1,000 years.
Graham Smith, its head, is one of several members who have reportedly been arrested while unloading a van.
48-year-old doctor James McDermott said he heard about in a group chat and said it was a "moment of authoritarianism."
The protesters gathered mainly in Trafalgar Square along the procession route next to a statue of Charles I, who was beheaded in 1649, leading to a short-lived republic.
In an interview with Reuters last week Smith said expected over 1,000 people dressed in yellow shirts to join the protest on May 6, where they will hold up placards, and give speeches.