Hundreds of yellow-clad demonstrators gathered among the 10-deep crowds lining the procession route in central London to stand out from those clad in red, white and blue, and to hold up signs saying “Not My King”.

Republic said its leader Graham Smith had been detained before the procession began and photos circulated on social media showing police officers seizing demonstrators’ placards.

“We absolutely understand public concern following the arrests we made this morning,” Commander Karen Findlay of the London Metropolitan police said in a statement.

“Over the past 24 hours there has been a significant police operation after we received information protesters were determined to disrupt the coronation procession.”

Republic had vowed to mount the biggest protest against a monarch in modern history and protesters booed as King Charles and Queen Camilla made their way to Westminster Abbey, and as the service was relayed publicly on large speakers.

“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.

They said they had acted because they believed protesters would seek to deface public monuments with paint and disrupt “official movements”.

“All of these people remain in custody,” Findlay said.

Police said in a separate statement on Saturday that they had arrested three people earlier in the day based on intelligence that protesters were planning to throw rape alarms at the procession which could have scared the horses involved and thereby caused a risk to public safety.

Amongst the items seized during the arrests in London’s Soho district were a number of rape alarms, the police added.

Although protesters were in a minority compared with the tens of thousands gathered 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 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,” Clive Lewis, an opposition Labour Party MP, said.

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

“Don’t you think this is all a bit silly,” one placard said.

Most of the anti-monarchy protesters 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.

-AAP