NEW YORK — When the stranger on the subway car unzipped his fly and started fondling himself, Thao Nguyen, 23, did what any woman confronted by a flasher might like to do.
She took out her cellphone, snapped him in the act with its built-in camera, then posted the image online. The smirking fellow on the uptown R train with his hands in his lap was displayed in the digital pillory of a photo-sharing website called Flickr.com as "pervert081805."
In this city of surveillance, where more than 5,000 video cameras monitor the mean streets, it was the perfect tale of tables turned, of public shaming in the Internet Age and the swift justice of the smart mob.
The incident prompted blog commentaries from Spain to Australia and a splash of front-page tabloid outrage. By Thursday, within a week of the photo being posted, a suspect had been arrested and charged with four counts of lewd behavior.
It was the third time this year that women had used cellphone cameras to expose flashers on the nation's largest subway system, until now Manhattan's last reserve of public anonymity.
Several analysts of social networks and online community considered the subway incident an empowering example of how people could take technology -- and justice -- into their own hands in an act of citizen journalism. Digital cameras and websites have given people an ability to distribute images widely that not so long ago was the province of professional news organizations.
"This is a fascinating phenomenon that evolved through the unexpected use of technology," said Cory Doctorow, European affairs coordinator for the Electronic Frontier Foundation who helps edit Boing Boing, a weblog with 1.3 million readers that had highlighted the subway photo. "It classically illustrates the way people find their own uses for technology."
Even as they applauded Nguyen's quick thinking, however, some scholars were troubled by the unintended consequences of a world in which almost no action seems to go unrecorded by cellphone cameras, spy cams and security monitors. It is the next step, they said, in the creeping "paparazziation" of society.
In the four years since camera-equipped mobile phones reached the market, the devices have changed the way many people look at the world. By the end of this year, 500 million people are expected to own a telephone with some sort of image-capturing device. That number could swell to 3 billion by 2010, according to cellphone manufacturer Nokia.
This year, New York announced plans to equip its entire subway system with cellphone service, as part of an anti-terrorism effort to safeguard the public transit system with 1,000 video cameras, 3,000 motion and perimeter sensors, intelligent video and closed-circuit television.
That will make it possible for subway riders -- be they amateur crime fighters or voyeurs -- to take and instantly distribute cellphone photos without leaving the underground trains.
In the fluid medium of the Web's 8 billion electronic pages, however, where digital photos are easily altered and deception is sometimes a form of entertainment, several analysts worried about the risks posed by online vigilantes.
An estimated 100,000 people saw the subway picture posted online. Several hundred posted messages about it. Most of them took the image at face value without much assurance the image had not been faked or its meaning altered.
"The flip side of the wisdom of crowds is lynching," said senior technology analyst Piers Young at Oxford University in England, who maintains a blog called Monkey Magic. "There seemed to be a whole load of people jumping on the bandwagon without much thought for the justice issues. It is easy to imagine similar cases that would be twisted."
Tama Leaver, who maintains a blog in Perth, Australia, called Ponderance, said, "In effect, in the so-called court of public opinion, the flasher was tried, found guilty, denounced and shamed, all without the normal mechanisms of the law having any substantial involvement.
"If this becomes a trend, and a camera-phone-enabled trial-by-Flickr gains an odd sort of credibility, the potential to abuse such a system is virtually limitless."
Although much of the online response to Nguyen's action was laudatory, Stowe Boyd, president of Corante, an online media company that operates 25 blogs with a monthly readership of 450,000, was taken aback by the hostility of some postings.
"It was astonishing, I thought, how many people took the time and the energy to attempt to discredit her claim," Boyd said. "When you get into controversial things, the trolls do come out."
Nguyen deleted the image from Flickr.com once a suspect had been arrested.
But first she messaged the online community: "It's easy to say that I should have done this or that. In reality, when it happened, I didn't know what to do. I was very nervous, but I knew I had to do something."