Police arrested the leader of the anti-monarchy group Republic hours before King Charles' coronation on Saturday, removing him from the few hundred yellow-clad protesters who had gathered among the crowds lining the procession route in central London.

Republic had said it would mount the biggest protest against a British monarch in modern history. Protesters wore yellow t-shirts to stand out from those clad in red, white and blue, and held up signs saying “Not My King”.

They spent most of the service booing or singing songs such as “he is just a normal man”.

But London police had warned they would take action if protesters tried to “obstruct the enjoyment and celebration” of the day, and they formed a ring around the group.

Republic said its leader Graham Smith had been detained on Saturday morning and a photo posted on Twitter showed him sitting on the ground surrounded by police officers.

"It is disgusting and massively over the top," said Kevin John, 57, a salesman from Devon who was among the protesters.

"It is also hugely counterproductive by the police because all it has done is create a massive amount of publicity for us. It is completely crazy."

Police did not confirm Smith's arrest but said they had arrested four people on suspicion of causing a public nuisance and three people on suspicion of possessing articles to cause criminal damage in what they called a“significant police operation”.

Republic said hundreds of its placards had been seized.

"As we speak the whole core team of Republic is still being detained," it said on Twitter.“They will probably be released when the whole monarchy pr-show is over.”

Protests also took place in Glasgow in Scotland and Cardiff in Wales, with signs held up saying:“Abolish the monarchy, feed the people.” On social media, many contrasted the cost of living crisis in Britain with the pomp and pageantry on display at the coronation.

Although the protesters were in a minority compared with the tens of thousands gathered on London's streets to support the king, polls suggest support for the monarchy is declining and is weakest among young people.

With the crown passing from Queen Elizabeth to her less popular son, republican activists hope Charles will be the last British monarch to be crowned.

It has a hereditary billionaire individual born into wealth and privilege who basically symbolises the inequality of wealth and power in our society,” said Clive Lewis, an opposition Labour Party lawmaker.

Staggeringly expensive

In London, protesters demanded an elected head of state. They say that the royal family has no place in a modern constitutional democracy and is staggeringly expensive to maintain.

Most of the anti-monarchy protesters on Saturday had congregated in Trafalgar Square next to the bronze statue of King Charles I, who was beheaded in 1649, leading to a short-lived republic.

Some held up signs saying “privatise them” and “abolish the monarchy, not the right to protest”.

Other signs featured a picture of Meghan, the wife of Charles' son Prince Harry, with the words “the people's princess”, and “God Save the King” with a picture of the late soccer great Pele.

Since Charles became king last September, there have been protests at royal events. He was heckled at a Commonwealth Day event at Westminster Abbey in March and targeted with eggs in York in November.

The death of the queen has also reignited debate in other parts of the world, such as Australia and Jamaica, about the need to retain Charles as their head of state.

The state government of New South Wales said it had decided not to light up the sails of the Sydney Opera House to mark the coronation, in order to save money.

While many other European monarchies have come and gone, or are far diminished in scale and importance, the British royal family has remained remarkably resilient.

In Britain, polls show the majority of the population still want the royal family, but there is a long-term trend of declining support.

A poll by YouGov last month found 64 per cent of people in Britain said they had little or no interest in the coronation. Among those aged 18 to 24, the number voicing little or no interest rose to 75 per cent.