A republican protester was arrested and put in the back of a police van after shouting "who elected the King", he claims.
Symon Hill, 45, was walking home from church when he stumbled across a proclamation ceremony for His Majesty in Oxford on Sunday afternoon.
He says he couldn't stomach it when he heard a speaker telling the crowd to accept the King as their "only lawful and rightful Liege Lord".
Moments after shouting out "who elected him" members of the public told Symon to "shut up" and security guards pushed him backwards, he says.
The republican was arrested by officers and held in a police van for around 30 minutes before being de-arrested.
He is now considering taking legal action against Thames Valley Police for unlawful arrest.
Symon told The Mirror: "The High Sheriff was reading out the proclamation and said we had to accept Charles as our only lawful and rightful Liege Lord.
"I find it hard to stomach in the 21st century that I should accept someone as my Liege Lord.
"I shouted out "who elected him?". Most of the people in the crowd didn't react. A couple of people told me to shut up - I thought fair enough that's their view. They weren't aggressive or anything.
"I then said a head of state is being imposed on us without our consent.
"Before I knew it there were three security guards in front of me telling me to be quiet. One of them was so close his nose must have been a couple of inches from my nose.
"They started pushing me backwards. Then I saw police pushing through the crowd towards me. They grabbed me and took me down the street.
"One of the police officers told me I was under arrest. I was shocked. I could hear people in the crowd asking why I was being arrested.
"Some people were glad I was arrested. But two men, who supported the monarchy, followed the police and said 'this is a free country and he has a right to express his view'."
Symon said he was taken away from the stage which had been set up outside Oxford's Carfax Tower for the proclamation.
His hands were cuffed behind his back and he was put in a police van by the officers.
They transported him to the nearby police station and then de-arrested him in the back of the van, before driving him back home.
Symon said: "I asked one of the coppers 'why have you arrested me' and he said you might have caused distress to one of the members of the public.
"I think it's a bit patronising to say people might have been distressed. I'd be very surprised if anyone there was distressed by what I said.
"Surely people can cope with hearing views they disagree with. If I can cope with hearing pro-monarchy views I think other people can cope with hearing a couple of sentences of a view they disagree with."
Symon has been inundated with messages of support since posting about his arrest on social media.
He added: "I've had a lot of messages from people saying they don't agree with my views about monarchy but agree with my right to free speech."
The part-time teacher said he was told by police he had been arrested under the Police, Crime, Sentencing and Courts Act, however officers have since said he was arrested under the Public Order Act.
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A spokesman for Thames Valley Police said: “A 45-year-old man was arrested in connection with a disturbance that was caused during the county proclamation ceremony of King Charles III in Oxford.
“He has subsequently been de-arrested and is engaging with us voluntarily as we investigate a public order offence.”
The spokesman confirmed the man was arrested on suspicion of an offence under the Public Order Act.
Symon is now considering taking legal action against Thames Valley Police for unlawful arrest.
"I think it could be important to discourage the police from acting like this again," he added.
