Demonstrations against the monarchy took place in London as well as Glasgow in Scotland and Cardiff in Wales.
Police arrested the leader of the anti-monarchist group Republic and 51 others at the coronation of King Charles III, saying their duty to prevent chaos outweighed their right to protest.
The Republic said its leader, Graham Smith, had been taken into custody on Saturday morning, and a photo posted on Twitter showed him sitting on the ground surrounded by police officers.
“We fully understand the public’s concern following the arrest we made this morning,” London’s Metropolitan Police Commander Karen Findlay said in a statement.
“Over the past 24 hours, there has been a major police operation after we received information that protesters were determined to disrupt the coronation procession.”
The Republic has said it will launch the largest protest against the British monarch in modern history. Protesters stood out wearing yellow T-shirts and holding signs that read “Not My King”.
Most of the time they’re booing or singing, like “he’s just a normal guy”.
Kevin John, 57, a salesman from Devon who was among the protesters, said: “It’s disgusting and it’s gone too far.”
“It also has a huge backlash against the police because all it’s doing is creating a lot of publicity for us. It’s totally insane.”
Police have not confirmed Smith’s arrest. They said they took action because they believed protesters would try to deface public monuments with paint and disrupt the “official movement”.
“All of these people are in custody,” Findlay said.
The Republic said hundreds of its placards had been confiscated.
Protests were also held in Glasgow, Scotland, and Cardiff, Wales, with signs reading “Abolish the monarchy, feed the people”. On social media, many compared the UK’s cost of living crisis to the pomp and pomp of the coronation.
Royal biographer Christopher Wilson told Al Jazeera that the king had seen protesters all his life and was used to demonstrations.
“I think he’s a true democrat who believes in free speech,” he said.
“The republican movement is by no means a terrorist movement. It’s just voices of protest and they have a right to free speech. After all, Britain is supposed to be the cradle of democracy. Free speech is one of the most important principles in our lives,” he added.
Although the protesters were a minority compared with the tens of thousands who gathered on the streets of London to support the king, opinion polls showed support for the monarchy was falling and was lowest among young people.
As the throne passes from Queen Elizabeth to her less popular son, Republican campaigners want Charles to be the last British monarch to be crowned.
While many other European monarchies have come and gone, or been greatly weakened in size and importance, the British royal family has remained remarkably resilient.
In Britain, opinion polls show a majority still want the royals to be royals, but support has long been on a downward trend.
A YouGov poll last month found that 64% of Brits said they had little or no interest in the coronation. Among 18- to 24-year-olds, that number rose to 75 percent who indicated little or no interest.
More than 11,000 police officers patrolled the coronation, the biggest ceremonial event in the British capital for 70 years.