The leader of an anti-monarchy group that had planned to protest King Charles III's coronation was released by police after 16 hours of detention.

Graham Smith had been arrested ahead of Saturday's festivities, with police saying that while they understood public concerns about the arrests, they had acted after receiving information that protesters were determined to disrupt the coronation procession. 

Smith denied any plans to sabotage the procession, saying his group planned a peaceful demonstration.

"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," he added.

Smith, whose group Republic advocates for abolishing the monarchy, was among more than 50 people who were detained using new powers rushed into law this week by the UK government to crack down on protests by direct action groups.

London police chief Mark Rowley had 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.

"These arrests are a direct attack on our democracy and the fundamental rights of every person in the country," Smith said. "Each and every police officer involved on the ground should hang their heads in shame."

Parties planned across Britain

King Charles said on Sunday that he was "deeply touched" by the celebration of his coronation, in what was in Britain's biggest ceremonial event in 70 years.

The king and queen were "profoundly grateful both to all those who helped to make it such a glorious occasion — and to the very many who turned out to show their support in such numbers in London and further afield," a Buckingham Palace spokesperson said.

Charles' heir Prince William and his wife Kate met with crowds in Windsor, ahead of the "Coronation Concert" in the evening, where some 20,000 members of the public and invited guests will gather.

The king's younger brother Prince Edward, his sister Princess Anne, and his nieces Princess Beatrice and Princess Eugenie also attended so-called "Big Lunch" events.

Even Prime Minister Rishi Sunak hosted a coronation lunch on Sunday, with the guests including Ukrainian families, youth groups and US first lady Jill Biden.

"From a cup of tea with a neighbor to a street party, a Coronation Big Lunch brings the celebrations to your neighborhood and is a great way to get to know your community a little better," Buckingham Palace said about the celebrations.

Monday has been declared a bank holiday in the UK.

jcg/dj (Reuters, AFP, AP)