The activists, who want to replace the King with an elected head of state, had been campaigning near the statue of King Charles I in Trafalgar Square, about a kilometre from Westminster Abbey and just south of the protest route.
The group, which said it had informed police of its plans to hold placards and chant “Not my king”, earlier said it was expecting about 1500 to 2000 protesters but it was unclear how many turned out.
On Saturday morning (Saturday evening AEST) several of the group’s members were stopped near Trafalgar Square and led to police vans.
“This morning, Graham Smith and five members of our team were arrested,” the group tweeted on Saturday, saying police “won’t say what for”.
“Hundreds of placards were seized. Is this democracy?”
The Metropolitan Police earlier said it had arrested four people in nearby St Martin’s lane on “suspicion of conspiracy to cause public nuisance” and another three on “on suspicion of possessing articles to cause criminal damage”.
“A number of arrests” were made of people suspected of breaching the peace.
Read Related Also: 'Our hearts are broken': Families mourn Australians killed in plane crash
The new rules, signed into law by the King on Tuesday, just days before the coronation, empower the police to take stronger action against peaceful protesters.
Long-standing protest tactics such as locking on, where protesters physically attach themselves to things like buildings, could lead to a six-month prison sentence or “unlimited fine”, according to the UK Home Office.
Republic said the Home Office sent the group a letter and asked for it to be forwarded to members, an act it interpreted as “a passive/aggressive intimidation of a legitimate protest group”.
Others managed to protest along the route, dressed in yellow and waving placards with slogans including “king parasite” and “abolish the monarchy”.
Police have said they would have a “low tolerance” for people seeking to disrupt the day, sparking criticism that they are clamping down on free speech.
Pomp and pageantry for coronation
– Reported with CNN and Associated Press