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-pound note.

Instead, a counter-campaign of online harassment, including threats of rape and death, against several high-profile women 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 website 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 featured among five historical figures, the social reformer Elizabeth Fry, would be replaced by Winston Churchill, indisputably male.

Surely, Criado-Perez argued, there were enough women of note in British history to find at least one more?

In case the bank lacked inspiration, she led a group of campaigners dressed up as the suffragist Emmeline Pankhurst, the novelist George Eliot (born Mary Ann Evans) and the Celtic warrior queen Boadicea, who fought Roman invaders in A.D. 60. Criado-Perez, 29, hand-delivered more than 35,000 signatures supporting her cause to the bank’s doorstep in London and collected nearly $20,000 in donations to mount a legal challenge should the "men of the bank" (there are no women on the nine-member committee that sets interest rates) ignore the 2010 Equality Act obliging public institutions to keep in mind the goal of gender equality in all matters that they decide.

The departing governor of the bank, Mervyn A. King, fond of pointing out that one woman, the queen, is on the back of every bill and coin, appeared to have little time for the debate.

But in July he was replaced by a younger man, a Canadian named Mark J. Carney, the first non-Briton to run the bank in its 319-year history. Carney seized the opportunity to make a gesture.

On July 24, Carney said that it had always been the bank’s intention to include another woman among the historical figures on the bank notes, and he announced that Austen would appear on future 10-pound notes. He also vowed to review the whole process of choosing historical figures for the notes.

"A brilliant day for women," Criado-Perez said in response.

But that same day on Twitter a trickle of abuse grew into a shower of crude rape and death threats against Criado-Perez at a rate of nearly one per minute. Several other women, from members of the public to members of Parliament, have also been the targets of Twitter attacks. Three female journalists received bomb threats.

"I’m going to pistol whip you over and over until you lose consciousness," one Twitter user warned Criado-Perez, threatening to "then burn ur flesh."

"I will rape you tomorrow at 9pm," a Twitter user told Stella Creasy, a Labour Party legislator. "Shall we meet near your house?"

Two men, ages 21 and 25, have been arrested so far in connection with the harassment. Scotland Yard’s electronic-crime unit is investigating the Twitter attacks involving mostly anonymous Internet users, so-called trolls.

What is perhaps most striking about the reaction, said Caitlin Moran, a columnist for the Times of London and the author of the witty 2011 feminist manifesto "How to Be a Woman," is how little it took to set it off.

"If even a small thing like this, a nice middle-class debate about putting Jane Austen’s picture on the opposite side of a bank note from the queen, causes a storm of abuse like this, what will happen when we get to the bigger issues?" Moran asked in a phone interview.

Twitter, under pressure after receiving an online petition with at least 124,000 signatures, announced Saturday on its site that it was introducing a new one-click button to report abuse on every post. The new feature will help users navigate their way to an online form. The company also pledged to dedicate more staff members to identifying abusive posts and is updating its rules, stating explicitly that it will not tolerate abuse.

Tony Wang, who runs Twitter’s British operation, posted a personal apology "to the women who have experienced abuse on Twitter."

The abuse is "not acceptable in the real world, and it’s not acceptable on Twitter," he said. He added, "There is more we can and will be doing to protect our users against abuse."

The debate highlighted a basic tension: technology has made female campaigners more vulnerable to sexist abuse, but it has also empowered them. (Criado-Perez collected her signatures on Change.org and has more than 24,600 followers on Twitter.)

Tanya Gold, a columnist for the British newspaper The Guardian, warned against asking social networking sites to "police our debate," suggesting that "misogynists on Twitter should be shamed rather than criminalized."

Carney and his colleagues at the Bank of England may be relieved to find that the attention of equality campaigners has for now shifted the focus away from the still overwhelmingly male world of finance.

If the bank note campaign was about stopping women from being "airbrushed" out of history, in banking "you don’t need an airbrush," said Niall Ferguson, the economic historian and Harvard professor, in an email. In the 19th century, he pointed out, female members of the powerful Rothschild banking family were never allowed to look at the books, much less play a part in managing the family firm.

Even today, only 6 percent of central bankers are women, Ferguson said. To be sure, as the debate in Washington surges around the candidacy of Janet Yellen to lead the Federal Reserve, there has never been a female governor of the Bank of England, which is nevertheless nicknamed "The Old Lady of Threadneedle Street."

But few could have foreseen that Austen, a writer perhaps best known for her musings on 19th-century romance, might inadvertently become a feminist symbol.

"She has a wide popular and a varied political appeal," said Devoney Looser, a professor of English at Arizona State University and an Austen specialist. "Unlike someone like Emmeline Pankhurst, it’s more difficult to slot Austen politically. She’s embraced by conservatives and progressives both."

© 2013 The New York Times Company