"How Did ABC, CBS, NBC, MSNBC and CNN Miss This Story?" was the headline on the full-page ads that ran Friday. The headline is superimposed over photos of the rally that drew tens of thousands of anti-tax demonstrators and Obama administration opponents.
Fox's taunt drew immediate reaction from the other networks, all of whom quickly offered up descriptions, transcripts of news reports and video proof of their rally coverage. Some denounced Fox for falsehoods.
CNN anchor Rick Sanchez put it even more bluntly in a heated, lengthy air-on retort that CNN, which competes directly with fellow cable channel Fox, showed repeatedly. Sanchez aired clips of CNN's coverage from last Saturday, including interviews with protesters and a wide shot of the massive rally that looked like one featured in the Fox ad - a shot Sanchez said was aired repeatedly by CNN.
"Let me address the Fox News Network now perhaps the most current way that I can, by quoting somebody who recently used a very pithy phrase, two words," Sanchez told viewers. "It's all I need: You lie."
Fox took out the ad in The Washington Post and in the Wall Street Journal and New York Post, two papers owned by Fox's parent company.
The network has taken a pugnacious approach before in its promotional efforts, but this latest move is unusually provocative even for Fox.
Fox defended its ads in an e-mailed statement from a marketing executive that called other networks on the carpet again for their coverage of the rally and events preceding it.
"Generally speaking, it's fair to say that from the tea party movement ... to Acorn ... to the march on 9/12, the networks either ignored the story, marginalized it or misrepresented the significance of it altogether," said Michael Tammero, vice president of marketing for Fox News.
ABC News spokesman Jeffrey Schneider said the ad was "just false" and that ABC has "a real problem with the Washington Post accepting an ad like this."
In an e-mailed statement, the Post said it doesn't reject an ad based on its content or sponsor unless the ad is "illegal, false, advocates illegal actions, or is not in keeping with standards of taste."
When an ad is not contrary to those standards "we will not place limits on speech or content," the paper said, adding that Fox was "expressing its opinion" about its competitors.
NBC declined comment but noted that NBC News had crews on the mall and the event led "NBC Nightly News" on Saturday and was covered on "Today" Sunday morning. MSNBC also covered the story with live reports throughout the day, the network said.