It’s good to be the king. But it’s not without its traps, as King Charles III learned last weekend when the organizers of his coronation invited millions of Britons to pledge an oath of homage to the monarch during the ceremony on Saturday.

“A spectacular misjudgment,” said Graham Smith, whose group, Republic, wants to abolish the monarchy. “Discordant and potentially tone deaf,” posted Anna Whitelock, an expert on the monarchy at City, University of London. “More like the stuff of a Stalinist people’s republic,” wrote the columnist Mick Hume.

The archbishop of Canterbury, the Most. Rev. Justin Welby, who will preside over the service, insisted that the oath would be purely voluntary. It was meant as a democratizing gesture: At Queen Elizabeth II’s coronation in 1953, only members of the hereditary aristocracy swore allegiance.

Such are the problems vexing Charles as he prepares for his coronation, Britain’s first in 70 years. In the seven months since he ascended the throne, royal watchers say, the new king has worked to make the monarchy more accessible, forward looking and inclusive. Yet the hoary rituals of the coronation are a reminder of how — in a secular, multiethnic, digital-age society — the crown is fundamentally an anachronism.