PARIS -- The Internet, it is sometimes said, turns every citizen into a journalist. If that is the case, some Twitter users in Britain are discovering one of the downsides of the business.
As many as 10,000 Twitter users reportedly face the threat of legal action because of comments posted on the Internet or forwarded to others in which they referred to a BBC report wrongly linking a former Conservative Party official to the sexual abuse of a child. The official, Alistair McAlpine, was not named in the Nov. 2 BBC report, but enough clues were provided that Twitter users were able to identify him -- which they did, in great numbers.
The BBC quickly settled a libel claim, paying Mr. McAlpine £185,000, or nearly $300,000, and apologized for the error as a case of mistaken identity. Another British television broadcaster, ITV, agreed on Thursday to pay Mr. McAlpine £125,000 to settle another claim, this one over a subsequent broadcast in which a list purporting to show Conservative figures linked to sex abuse accusations had been visible to viewers.
Mr. McAlpine did not stop with the mainstream media. On Friday, a spokeswoman for the politician told The Guardian newspaper that his lawyers had identified 20 "high-profile tweeters" from whom they were seeking libel damages. Among them were a comedian, Alan Davies; Sally Bercow, the wife of John Bercow, the speaker of the House of Commons; and George Monbiot, a Guardian columnist.
There have been previous libel suits over comments posted on Twitter, a site that lets users write short messages to their followers. In March, a New Zealand cricket player, Chris Cairns, was awarded £90,000 by the High Court in London over a Twitter post by an Indian cricket official in which he falsely accused Mr. Cairns of match-fixing.
But the campaign by Mr. McAlpine appears to be the broadest yet, and it employs some novel tactics.
"Many people have had their reputations trashed on Twitter before, but nobody has decided to take action on this scale," said Tim Lowles, a media lawyer at Collyer Bristow in London.
In addition to the prominent figures, Mr. McAlpine is reportedly pursuing action against thousands of other Twitter users, including people who had merely repeated to their own followers comments made by others.
Twitter users with fewer than 500 online followers who are "wishing to apologize and make contact" can use a Web site created by Mr. McAlpine's law firm, R.M.P.I., to try to settle their cases.
"It is not this firm or Lord McAlpine's intention to create any hardship," a letter posted on the site states.
All that Twitter users who think they might have libeled Mr. McAlpine have to do is read the letter, fill out a downloadable form asking them for details of the postings in question, make an apology and send the form back via e-mail. Those who respond are reportedly asked to make donations to charity, and the firm warns that there will also be a "small administrative charge."
British libel and defamation laws are notoriously friendly to claimants. While a planned overhaul would give publishers a bit more protection, the rise of social media like Twitter has vastly expanded the possibilities for libeling someone.
Some of the Twitter posts in question do not directly mention the offense to which Mr. McAlpine was falsely linked. The posting by Ms. Bercow asked, "Why is Lord McAlpine trending? *innocent face*" She insisted in a subsequent post that her initial comment had not been libelous.
Mr. Lowles said that under British law it was possible to libel someone indirectly or by innuendo, even if the incorrect accusations were not mentioned directly, as long as they were clear from the context.
It is not clear how far Mr. McAlpine intends to go in pursuing Twitter users who commented on the BBC accusations. R.M.P.I. did not return calls.
Some people register on Twitter anonymously or under fake names. British courts have ruled in previous cases that Internet companies like Twitter can be ordered to turn over the personal details of users, but this can be time-consuming and costly. A Twitter spokeswoman in Britain declined to comment on whether the company had received any such requests.
Like other social media companies, Twitter considers itself a conduit for its users but disavows responsibility for the content of the 400 million comments posted daily. The site's guidelines state that users bear this burden.
"Most people still don't think when they write something on Twitter that they are actually publishing," Mr. Lowles said. "Whether or not this will act as a deterrent, I don't know. People should think before they tweet."
Could the threat of libel action dissuade Britons from posting on Twitter? By Friday, at least one prominent Twitter account had disappeared -- that of Ms. Bercow, who had more than 50,000 followers, and who had followed up her comments on Mr. McAlpine with an apparent mistake in a different case: she appeared to have violated a court order by naming a British teenager who had been abducted by one of her teachers.
"This could have a chilling effect," Paul Bernal, a lecturer in media law at the University of East Anglia, said of the McAlpine case. "I know people who have said that they are not going to post as much because of this."
This article originally appeared in The New York Times.