Several anti-monarchist protesters including the leader of the UK’s most prominent republican group have been arrested in the hours leading up to King Charles’ coronation ceremony.

Chief executive of Republic Graham Smith was detained by police in central London when collecting drinks and signs for protesters based in Trafalgar Square, the Guardian reports.

London’s Met police announced on social media this week that they would have a “low tolerance” for anyone planning to “undermine” the event.

Director at Republic, Harry Stratton, told the outlet: “They were collecting the placards and bringing them over when the police stopped them. The guys asked why and they were told: ‘We will tell you that once we have searched the vehicle.’ That’s when they arrested the six organisers.

“We asked on what grounds they had been arrested but they wouldn’t say. It is a surprise as we had had a number of meetings with the police. They had been making all the right noises.”

The arrest came at about 7.30am local time as hundreds of protesters gathered in Trafalgar Square wearing yellow T-shirts and waving flags.

The Herald’s lifestyle and entertainment editor Jenni Mortimer, reporting from the scene in London, said the protesters appeared to have delayed the start of the ceremony.

 “[There is] lots more security and police going closer to the procession line,” she said.

“Crowds are cheering as guards have moved on protesters from The Mall. The group appeared to be protesting the monarchy and had signs relating to abolishing the monarchy.

“Police have gone back to their places and a guard could be heard saying ‘if these protesters keep protesting, the coronation will be delayed ever further’.”

A few thousand protesters are expected to gather below Nelson’s Column by midday, representing the Dutch, Swedish and Norwegian republican groups. Those who gathered early on Saturday morning UK time were outnumbered by those celebrating the event and by law enforcement.

In the largest police operation the UK has seen, 11,500 police officers are present for the procession.

The man who was found guilty last month of a public order offence for throwing five eggs at King Charles during a visit to York has also made an appearance.

Patrick Thelwell, 23, said he has “absolutely not brought any eggs” this time.

“My parole officer tells me counter-terrorism is following me. I am sure that I will be arrested anyway. It will be interesting to see how far we have descended into being a fascist country.”