The Metropolitan Police has expressed “regret” over the arrests of six anti-monarchy protesters on Coronation day.
Republic chief executive Graham Smith, who was among the group, said he has now received a personal apology from police officers.
He said he did not accept the apology and would take legal action after no charges were brought against him.
The Met also confirmed it had used a controversial new law to detain the group.
Mr Smith said a chief inspector and two other officers visited his Reading home on Monday evening to issue the apology.
He told PA news agency: “They seemed rather embarrassed to be honest.
“I said for the record I won’t accept the apology. We have a lot of questions to answer and we will be taking action.”
Mr Smith earlier said he wanted a “full inquiry” into the “disgraceful episode”.
The Met said a review found there was no proof the six protesters, who were detained when their vehicle was stopped near the procession route, were planning to “lock on”, a protesting tactic which is now banned.
Recent changes to the law, passed last week, make it illegal for protesters to use equipment to secure themselves to things like railings.
The Met said the group of six were detained after items were found in a vehicle which officers “had reasonable grounds to believe could be used as lock on devices”.
But the force said it was “unable to prove intent to use them to lock on and disrupt the event”.
One man in the group was also arrested for possession of a knife or pointed article.
The Met said it was “not clear at the time” to the arresting officers that “at least one of the group stopped had been engaging with police” about holding a lawful protest prior to the Coronation.
“We regret that those six people arrested were unable to join the wider group of protesters in Trafalgar Square and elsewhere on the procession route,” a statement continued.
Now it has said all six people have had their bail cancelled and confirmed no further action will be taken.
Mr Smith said earlier on Monday that he had spent months consulting with officers about his group’s protest plans, and said in a statement on Twitter that his group would be “speaking to lawyers about taking legal action”.
He said he had been held for 16 hours on the morning of the Coronation after being stopped by officers who suspected him and group members of carrying “lock on” devices to tie themselves to inanimate objects.
“They also said they had intelligence, which is untrue,” he told BBC Radio 4’s Today programme.
“If they did have intelligence their intelligence officers are either lying or incompetent because there was never any discussion, thought, email, message, anything that suggested any intent to do anything disruptive.”
Mr Smith added that, after months of discussions with the Met, the force had “repeatedly said, right up until Friday, that they had no concerns about our protest plans, that they were well aware of what we were going to do and they would engage with us and not disrupt us”.
He continued: “So they’ve repeatedly lied about their intentions, and I believe they had every intention of arresting us prior to doing so.”
Mr Smith also rejected suggestions his arrest, along with other protesters, was necessary to limit disruption to the Coronation.
Shadow housing minister Lisa Nandy said “clearly something has gone wrong” in the handling of Mr Smith’s case, and expressed her support for a review into the matter, which has been requested by the Mayor of London, Sadiq Khan.
However, she said Labour was not committing to “wholesale repeal” of the new law introduced by the Conservatives last week, which has been criticised for clamping down on the rights of peaceful protesters.
She told BBC Breakfast: “One of the questions we have is ‘why was it that this group were clearly in contact with the Met, had informed them about their plans, and yet still ended up arrested up and prevented from protesting?’.
“If there is a problem with the legislation, of course we’ll rectify that in government, but we’re not into wholesale repeal of legislation without understanding what the actual problem is first.”
Earlier, Met Police Federation chairman Ken Marsh said officers “police without fear or favour,” insisting the force had done “an incredible job” policing the Coronation.
He told the Today programme: “We have to take into consideration everything that at that moment is put in front of us. If individuals intend to cause an incident which will affect others near them or around them… then we take action to deal with it.
“Protesting can take place in this country, but it’s to the level of which you perform that protesting that we have to balance and deal with.”