Anti-monarchists see an opportunity to prove their point in the coronation events.

While some Britons prepared for King Charles III’s coronation by buying royal paraphernalia or cooking for street parties, a 21-year-old student in the northern city of Leeds instead ordered 50 beach balls bearing the words “No more royals.”

The plan is to throw them around at a protest at Trafalgar Square in central London on Saturday organized by Republic, a group representing Britain’s anti-monarchist movement, which its members say is being energized by the coronation.

“The coronation does a lot of good for the movement just by being itself,” the student, Imogen McBeath, said in an interview. “Absolutely ridiculous.”

During the events surrounding the death of Queen Elizabeth II last year, the British republican movement laid low, wary of appearing insensitive at a time of mourning. But with attention again turned to the royal family, the anti-monarchists of Republic, whose thousands of members range from their teens to their 90s, have embraced a new strategy.

They said they expected at least 1,000 people to turn up for Saturday’s protest, wearing yellow, holding banners and chanting, “Not my king.” Several anti-coronation parties are also planned around the country, with members eager to use the crowning of King Charles as evidence of the absurdity of having a monarchy in this day and age.

“They will put a glittery golden crown on his head in a Christian church,” said Matt Turnbull, a 35-year-old Republic member who lives in London and planned to attend the protest, in an interview. “Look at it, and just accept that something about this feels weird in 2023.”

Mr. Turnbull said that he expected the coronation to make his stomach turn, but that it also felt good that he would not be alone in having such a feeling. “The worse it makes me feel to watch it,” he said, “the more quickly we will move to abolish it.”

That Charles appears to be less popular than Elizabeth, his mother, is also rising the hopes of anti-monarchists. Although 58 percent of respondents in a recent poll by YouGov commissioned by the BBC said they still preferred a monarch to an elected head of state, the figures also suggested that a change may be underway, with only 32 percent of people aged 18 to 22 backing the idea.

Riz Possnett, 19, a University of Oxford student who uses they/them pronouns, said that the monarchy and its colonial legacy were an outdated symbol for modern, multicultural Britain.

“The British identity can come from better places than an unelected king,” they said. “The coronation reminds how weird and archaic our system is.”

They and Mx. McBeath, who also uses they/them pronouns, said they had once shown their disdain for the monarchy by sneaking into the King’s Bed in Windsor Castle, a building that can be visited as a tourist attraction, making out there and reading Prince Harry’s autobiography in protest.

They said the coronation would be a key moment to highlight the idea that the only reason Charles will have a dedicated party and public holiday is that he was born into the right family — especially as many people in Britain are struggling to afford food and electricity.

“I think the pomp and ceremony of that all, the king wearing a crown, will feel like a slap in the face to people struggling,” said Mx. Possnett.

After the organizers of the coronation invited millions of Britons to pledge an oath of homage to the monarch and his descendants — a suggestion that drew swift criticism from many quarters — a friend of Mx. McBeath’s wrote an alternative pledge. “Pledging allegiance to someone and all their children is not a democracy,” said Mx. McBeath.

The alternative pledge swears allegiance “to the living Earth and its People; not any nation state or Monarch. I will uphold the values of Democracy, Solidarity, Justice, Peace and Love.”

Mx. McBeath said they planned to attend the protest at Trafalgar Square on Saturday to listen to speeches, sing and chant.

“My goal is to have more fun than all the monarchists around,” they said.