The whittling away of hard-won freedoms is especially troubling, activists say, because the social media have become the newest outlets for rebellion, replacing the street battles of the 1980s that forced the end of decades of dictatorship.

“New media and social networking services like Twitter have emerged as new political tools for antigovernment and left-wing people,” said Chang Yeo-kyung, a free-speech activist. “The government wants to create a chilling effect to prevent the spread of critical views.”

That accusation has been echoed by some international observers. The United Nations special rapporteur on freedom of expression was alarmed enough last year to lecture officials on the necessity for public scrutiny in a democracy.

And this year, Reporters Without Borders listed South Korea as a country “under surveillance” in a report titled “Enemies of the Internet,” putting it in the company of Russia, Egypt and other nations known for their intolerance of dissent.

The group said South Korea had intensified its longstanding campaign on material that appears to support North Korea. But the report said “censorship is also focused on political opinions expressed online — a critical topic in this election year.”

The government denies trying to stifle criticism and says it opens most cases after being alerted by citizens, including those who have deputized themselves as “cybersheriffs.”

In a statement defending its stance, the government said it acted because “character assassinations and suicides caused by excessive insults, the spreading of false rumors and defamation have all become social issues.”

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But the Rev. Choi Byoung-sung, a critic of the government’s environmental policy, argues that free speech is being undermined.

“They are burning down an entire house under the pretext of killing a few fleas,” said Mr. Choi, who fought the removal of his blog postings warning of potential health risks from cement containing industrial waste. (He won.)

South Korea’s government-supported love affair with the Internet has paid off: the country has some of the world’s fastest download speeds. And it is a point of pride that Seoul’s subway riders can surf the Internet with their smartphones.

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But with such obvious advantages for business came the unexpected: an onslaught of challenges to social mores. The aversion to challenging superiors had been so deeply ingrained that when South Korean airlines suffered an unusual number of crashes in the 1990s, investigators often partly blamed the hesitance of co-pilots to second-guess pilots even if an error might have been obvious.

The distance and anonymity of Internet communication wiped away many such fears. Suddenly, people who could not imagine using anything but polite honorifics to address those above them in the social pecking order let loose, criticizing leaders in ribald language normally confined to conversations with friends. The humiliation of those so boldly criticized, analysts say, is hard to overestimate.

“A tremendous emphasis is placed on the importance of upholding the public face,” Ms. Chang said.

Park Kyung-sin — one of the few members of the government’s Internet regulatory board appointed by opposition parties, and an ardent critic of its policies — says members of the political elite feel especially threatened because they see themselves as “fatherlike figures.”

One of the first decisions the board made after Mr. Lee came to power was to “purify the language” used against Mr. Lee because, as one commissioner later said, he should be treated like the father of the state, “an extended form of a family.”

Such socially conservative arguments had won less traction under Mr. Lee’s predecessor, Roh Moo-hyun, who was more accepting of criticism on the Web, in part because he was determined to abolish what political analysts called an “imperial presidency” and considered Web commentary generally friendlier than that in the conservative mainstream media.

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Under Mr. Lee’s appointees, regulators more than tripled the number of Internet posts removed or blocked, to over 53,000 last year from 15,000 in 2008, for infractions that include posting pornography, using profanity or supporting North Korea.

Government critics said the heightened surveillance began early in Mr. Lee’s term, after his government accused political enemies of using the Web to organize mass demonstrations in 2008 against a decision to import American beef.

Prosecutors were accused of reaching back to a dictatorship-era law when they indicted several of those held responsible for spreading “false rumors.” Among those charged: a teenager who sent text messages suggesting that students nationwide cut classes to join the protests. (He was acquitted.)

That law was ultimately ruled unconstitutional. But activists say the government has plenty of legal tools to fall back on, most notably a defamation law they say stretches the definition of the crime well beyond what would be accepted in other countries.

“Many criminal defamation suits are filed for statements that are true and are in the public interest,” said Frank La Rue, the United Nations’ special rapporteur, in his report last year.

For Mr. Park, the censorship board dissident, one of the worst problems is that his commission can act with impunity, often deleting content without notifying the author.

The board says it is working to become more transparent. But Song Jin-yong, whose account was blocked because he used a pseudonym that translated to “Lee Myung-bak bastard,” said the board was missing the bigger point about democratic rights.

“The government says I cannot even choose my own Twitter ID,” Mr. Song said recently. “Isn’t it part of my right to bad-mouth the president when I am unhappy with him?”