Two protesters who expressed republican sentiments have been arrested at events proclaiming the accession to the throne of King Charles III.

A man said he was arrested for shouting, “Who elected him?” when the proclamation was read out in Oxford.

Symon Hill, 45, said he had come across the event by chance as he walked home from church. The history tutor said that after he shouted the words, some people nearby told him to “shut up” and he responded by saying: “A head of state has been imposed on us without our consent.”

Three security guards approached him before police intervened, he said. Hill said he was taken to a police van, despite the protestations of others, who defended his right to free expression.

Hill said he had previously been arrested for engaging in non-violent protest against the arms trade but that he had not been surprised, despite believing his actions were justified.

In contrast, referring to his latest arrest, he said: “I don’t think I’ve ever seen anyone arrested on such threadbare grounds, let alone experienced it myself.

“I didn’t in any meaningful sense disrupt the ceremony; I just called out something that a few people near me would have heard, and then they carried on with the ceremony, and they [the police] collared me. I find it really alarming that you can be arrested for expressing an opinion in public. I am feeling quite shaken.”

After he wrote about the incident on Twitter, Republic, which campaigns to replace the monarchy with an elected head of state, wrote: “That’s outrageous.”

A Thames Valley police spokesperson 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 man was arrested on suspicion of a public order offence [under section 5 of the Public Order Act 1986].”

Section 5 applies to behaviour deemed likely to cause harassment, alarm or distress.

In Edinburgh, a woman holding a sign saying, “Fuck imperialism, abolish monarchy” was arrested moments before the reading of the proclamation. The incident took place outside St Giles’ Cathedral, where the Queen’s coffin is due to lie on Monday.

A police spokesperson said a 22-year-old woman had been arrested in connection with a breach of the peace. Some people were heard booing at the proclamation of the King during the Edinburgh event.