The head of the UK’s leading republican movement and five other organisers of an anti-monarchist protest at the coronation have been arrested on King Charles III’s procession route.

Graham Smith, the chief executive of Republic, had been collecting drinks and placards for demonstrators at the main site of the protest on Trafalgar Square when he was detained by police on the Strand in central London.

The group had been walking behind a rental van full of hundreds of placards when they were stopped by police. The Met police had tweeted earlier this week that they would have a “low tolerance” of those seeking to “undermine” the day.

Harry Stratton, a director at Republic, who arrived as Smith and the others were detained, said: “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, at about 7.30am, came as hundreds of anti-monarchist protesters had gathered at Trafalgar square with large flags and wearing yellow T-shirts as they looked to catch both the eye of the world’s media and a king on his coronation day.

Positioned by London’s oldest statue of Charles I, who lost his head to republicans nearly 400 years ago, those preparing to dissent as Charles III’s coronation procession passed them conceded they were “heavily outnumbered”.

As many as 2,000 protesters are expected to assemble below Nelson’s column by midday including representatives of the Swedish, Dutch and Norwegian republican movements

But the small huddle present early on Saturday morning, under an ominous grey sky, were surrounded by both those out to celebrate and the columns of police officers marching into their positions.

Maria Gomez, 39, from Boreham Wood, in Hertfordshire, said the protest, organised by the longstanding anti monarchy group, Republic, would be peaceful and she hoped to avoid clashes with those who had come to celebrate.

“Some were shouting ‘burn the yellow flags’ earlier but we have had intimidation before – there were eggs thrown in 2012,” she said. “People can get very angry with republicans.”

Radical Haslan, 29, a student who had come from Manchester, said he would be “staying safe with the others” at Trafalgar square, but some were planning to stand in other positions along the procession route “to make our feelings known”.

The presence of 11,500 police officers, the largest police operation ever seen in the UK, was not entirely reassuring to all of those who had gathered to make their opposition to the monarchy felt.

The protest has drawn all the better-known characters of the anti-monarchy movement, including Patrick Thelwell, 23, who was found guilty last month of a public order offence for throwing five eggs at the king during a visit to York.

“I’ve absolutely not brought any eggs”, he said. “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.”

Those assembling at Trafalgar planned to boo and wave their luminous yellow flags the king’s procession moved by on the way to Buckingham Palace from Westminster Abbey

Speeches were expected from the Labour MP Clive Lewis, the human rights activist Peter Tatchell and Paul Powlesland, the lawyer who was threatened with arrest when holding up a blank piece of paper after the death of the queen.

Imogen McBeath, 21, a campaigner linked to the No More Royals group, who say their message is a “declaration from the young, the queer and the dismissed”, said they would not seek to provoke any reaction but wanted to make a point peacefully.

The group has targeted statues in the past and McBeath made headlines when, with her partner, Riz, they jumped the security rope during a tour of Windsor Castle and sat on a bed first constructed for King Charles II in 1670 to kiss, eat junk food and read Prince Harry’s book Spare.

“This is the biggest monarchist event and so we had to be here,” she said. “But we don’t want to provoke anyone.”

Loretta Caughlin, 56, a restaurant owner from Penzance, in Cornwall, said she had been a member of Republic for 17 years. “It was in 2006 that I discovered Republic and realised I’m not the only one. There are a lot of us.”

Daniel Boffey Chief reporter