LONDON — It was a genteel campaign to ensure that British bank notes would continue to carry images of women, and it could have ended with the announcement last month that Jane Austen, the much-beloved novelist, would replace Charles Darwin on the £10 note.

Instead, a countercampaign of online harassment, including threats of rape and death, against several high-profile women here turned so nasty that Twitter took steps this weekend to tighten its global policy on reporting abuse.

There has been plenty of pride, but also a good dose of prejudice, as a small band of feminists in period costumes has initiated a national debate about power, rape and the limits of free speech in the age of social media.

Caroline Criado-Perez, a blogger and co-founder of the Web site The Women’s Room, began her campaign months ago when she realized that soon there might be no women — except Queen Elizabeth II, of course — left on British bank notes. The issue seemed urgent: in April, the Bank of England had announced that the only woman currently featured among five historical figures, the social reformer Elizabeth Fry, would be replaced by Winston Churchill, indisputably male.