The head of the anti-monarchy campaign group Republic was detained during a protest in Trafalgar Square ahead of King Charles' coronation.
Footage shows protesters wearing "Not My King" T-shirts, including Republic president Graham Smith, detained.
Six demonstrators, including Smith, were stopped while lowering banners near the parade route, the Republic said in a statement.
London Police confirmed multiple arrests were made at demonstrations in the capital.
Police said they had seized "locking devices" that demonstrators could use to secure themselves to guard rails.
Republic said police "misinterpreted" the straps used to secure the banners.
Campaign groups and human rights groups described the detentions as "incredibly worrying" and described them as "something you would expect to see in Moscow, not London".
Hundreds of protesters gathered in the rain in central London chanting slogans such as "Down with the Crown" and "Get a real job".
Other protests were also held in Cardiff, Glasgow and Edinburgh. No arrests were reported outside of London.
Republic posted on Twitter photos of police officers taking information on the detainees.
"The right to peaceful protest is up to this point," the group said, adding that the police would not disclose the reasons for their arrest.
The Metropolitan Police did not confirm the number of those detained, but said a series of arrests had been made near the Coronation route.
On Wednesday, law enforcement warned demonstrators that they would apply an "extremely low threshold" to protests during the coronation celebrations.
"The reports of people being arrested peacefully protesting the coronation are incredibly alarming," said Yasmine Ahmed, UK Director of Human Rights Watch.
A representative of the Republic told BBC Radio Leeds on Friday that they "absolutely had no intention of disrupting" the ceremony.
"We will be very visible, our voices will be loud, we will be hard to miss, but the procession and coronation plans will continue uninterrupted by us."
"Is this democracy?" Republic said Saturday morning that hundreds of their banners were seized and questioned. shared your message.
Prior to the ceremony, thousands gathered to watch the coronation, which stretched from Buckingham Palace to Westminster Abbey.
During the coronation, which was broadcast over loudspeakers in Trafalgar Square, hundreds of protesters booed "God Bless the King" announcements.
Around 300 people gathered for the Republic-sponsored protest in Cardiff City Centre.
While pro-independence supporters chanted anti-monarchy slogans during their march in Glasgow city centre, Our Republic group, which campaigns for an elected head of state, also held a separate rally.
As many as 13 protesters from climate change campaign group Just Stop Oil have been detained in a London shopping centre.
Just Stop Oil said that all five demonstrators were arrested in front of the Prime Ministry building.
A spokesperson for the group said their plan was to "just display T-shirts and a flag", adding, "This is a dystopian nightmare."