The leader of anti-monarchy group Republic and other members were released from custody after hours of detention during Saturday's coronation of King Charles that raised questions over whether the police response had been proportionate.
Police arrested the leader of Republic, Graham Smith, and 51 others in central London while thousands of royal fans were gathering in the streets for King Charles's coronation on Saturday, saying their duty to prevent disruption outweighed the right to protest.
Republic said that detained members began to be released late on Saturday evening, after nearly 16 hours in custody.
"I'm now out of the police station... Make no mistake. There is no longer a right to peaceful protest in the UK," Smith said on Twitter.
"I have been told many times the monarch is there to defend our freedoms. Now our freedoms are under attack in his name."
The police said on Saturday that they understood public concern following the arrests, but said they acted after receiving information that protesters were determined to disrupt the coronation procession.
London police chief Mark Rowley warned on Friday that police would take action if protesters tried to "obstruct the enjoyment and celebration" of people, saying there would be a "very low tolerance" for disruption.
In a separate incident, there was a dispute after police arrested three people early on Saturday morning and seized a number of rape alarms.
The police cited intelligence that there were plans to disrupt the procession with the alarms, but the local Westminster Council expressed concern that trained volunteers for a night safety scheme had been detained.
"We are deeply concerned by reports of our Night Stars volunteers being arrested overnight," local councilor Aicha Less said, adding the volunteers were being offered support.
"We are working with the Metropolitan Police to establish exactly what happened."
Wes Streeting, a senior lawmaker from the opposition Labour Party, declined to comment on specific arrests while investigations were ongoing but said London's police would have to be accountable as to whether their overall approach to the coronation was proportionate.
Tens of thousands of people turned out to catch a glimpse of the newly crowned King Charles and Queen Camilla, who rode in a state coach back to Buckingham Palace after Saturday's service at Westminster Abbey.
Not everyone who came to watch was there to cheer Charles, with hundreds of republicans booing and waving banners reading "Not My King".
Culture Minister Lucy Frazer said she had huge confidence in the police and added that they were right to take a tougher line on an event that could have raised questions about national security.
"I think overall (the police) managed to get that balance right," Frazer told Sky News.