British police arrested Graham Smith, leader of an anti-monarchist group Republic, ahead of King Charles' coronation on Saturday, a spokesperson for the group said.
London police chief Mark Rowley had warned on Friday that there would be a "very low tolerance for disruption" on the streets in central London where tens of thousands of people have gathered to watch the royal processions.
Police did not confirm the arrest, saying they did not immediately name those arrested.
A photo posted on Twitter showed Smith sitting on the ground surrounded by a group of police officers. An officer at the scene near Trafalgar Square said three republican protesters had been arrested for carrying paint.
More than 11,000 police officers are on patrol for the coronation, the biggest ceremonial event staged in the British capital for 70 years.
Rowley said ahead of the event that police would take action if protesters tried to "obstruct the enjoyment and celebration" of a significant number of people.
Hours before the historic event, in which the UK is about to witness the first coronation of a monarch in nearly 70 years, the organizers of anti-monarchy protest were arrested, the Republic group and Alliance of European Republican Movements (AERM) said on Twitter.
In video footage shared by the AERM, Graham Smith, the chief executive of Republic, and Ben Clinton, an organizer from the group, can be seen being arrested by the police.
In response to a question posed by the AERM, police confirmed that they are under arrest but gave no further details.
"This morning, @GrahamSmith_ and 5 members of our team were arrested. Hundreds of placards were seized. Is this democracy?" Republic said on Twitter.
Amid the coronation, hundreds of people are expected to hold a "Not My King" protest at the iconic Trafalgar Square in central London.
On early Saturday, Republic, known as the leading anti-monarchy group in the UK, wrote on social media: "Instead of a coronation we want an election. Instead of Charles we want a choice. It's that simple."
Charles, who took the throne last September following the death of Queen Elizabeth, the long-serving monarch, is expected to receive his crown in a service beginning at around 11 a.m. local time (1000GMT).
He will wear St. Edward's Crown, made of solid gold, while Queen Camilla will wear a modified version of the Crown of Queen Mary, fashioned in 1911.
After his coronation, Charles will be the 40th monarch to take the traditional Oath of Faithful Service.
