Heavily wired South Korea is policing its Internet in new and aggressive ways, giving citizens less room to say what they want about their leaders, their society and, in some instances, their combative neighbor to the north.

The tactic reflects a government that has trained itself to stay on guard for threats. Officials here describe South Korea as a democracy in perpetual defense mode, needing special measures to maintain order on an ideologically divided peninsula — one that becomes even more volatile after North Korean leader Kim Jong Il’s death.

“We need to preserve social order,” said Han Myeong-ho, an executive manager at South Korea’s Internet watchdog commission.

For South Koreans, 80 percent of whom use high-speed Internet, government meddling has long been a part of Web surfing: Anybody here who tries to visit an official Pyongyang Web site, for instance, will be redirected to a warning message, explaining that the site is blocked by government regulations.

But in several recent cases, critics say, South Korea has gone too far, cracking down on speech that wouldn’t draw attention in most democracies.

This week, South Korea’s Supreme Court upheld a ruling against one of the country’s most popular political commentators, who co-hosts a podcast that criticizes President Lee Myung-bak. The court said Chung Bong-ju, 51, was guilty of spreading rumors about Lee’s connection to an alleged stock fraud. Chung faces a one-year jail term.

“In America, it’s almost impossible to prove defamation against a public figure,” Chung said in an recent interview, before the Supreme Court determined his case. “Here it’s easy. . . . When people open their mouths now, they are regulated.”

The Internet watchdog

Largely, the conservative South Korea tightened regulations by using existing laws — ones that previous administrations found little use for — in more heavy-handed ways.

South Korea’s Internet watchdog, the Korea Communications Standards Commission, was created in 2008, empowered to patrol the Web for obscenity, defamation and anything that threatens national security. It’s technically an independent organization, but its nine members are appointed by the president.

One U.N. official, after a trip to South Korea in May 2010, said the KCSC “essentially operates as a censorship body.”

The KCSC doesn’t directly carry out the blocks, but its recommendations are almost never rejected; Internet service providers face large fines if they don’t comply, and message board operators can be jailed.

Three years ago, South Korea blocked some 2,000 Web sites on the grounds they threatened national security; it now blocks more than 80,000.

Just weeks ago, the KCSC created a team to monitor social networks such as Twitter and Facebook. After Kim’s death, some South Korean Web users posted tweets wondering whether they’d be punished for expressing condolences. The Justice Ministry said such messages would not violate the law.

No room for mockery

South Korea has become particularly sensitive about criticism of its politicians, as shown in the case of Song Jin-yong, 41, a financial worker in Seoul. In June 2010, Song created a Twitter account that he used almost exclusively to ridicule the president. The account’s name was part of the attack: It coupled Lee’s nickname (“2MB”) with a sound-alike reference to a common Korean curse word.

But this year, the KCSC blocked access to Song’s Twitter account, saying the account’s name “disgusts the general public.” Police took up the case, alleging, in one report, that Song used his account to “harm Lee Myung-bak’s social reputation.” Song faces an $850 fine.

Song had long considered himself apolitical, but with Lee in office, he became frustrated with some of South Korea’s most ­talked-about problems: a widening income gap, rising household debt, government corruption. He told as much to the KCSC panel several months ago, when he headed to a meeting room at its headquarters and officially appealed the decision to block his account.

The account name, Song told the panel, was a “creative” reference to a swear word — but not a swear word itself.

“The president has been selected by the majority of the nation,” the KCSC panel’s vice chairman, Kwon Hyuk-bu, told Song, according to an official transcript of the meeting. “It is normal for people to avoid swearing against the president.”

“I think everybody has the right to mock the president and criticize,” Song replied.

The KCSC denied Song’s appeal. Only one member expressed concern about the decision.

By most measurements, South Korea reflects one of the world’s most successful — and rapid — transformations, moving in 25 years from a military dictatorship into a model democracy. Almost half of its 48 million people own smartphones.

But the country’s older generations follow the codes of their youth: Criticism of the country’s decision-makers remains almost unheard of. South Korea’s three dominant national newspapers skew conservative, the difference only in degree. The government meddles in the hiring of some television executives, whose networks follow a similar line, media experts and politicians here say. The result is that South Korea’s mainstream news media provide one narrative about the nation, and Web surfers embrace an opposite one — a more critical one — online.

A 2011 U.N. report about freedom of expression in South Korea describes an “active and vibrant” Internet culture that is stifled by several vague laws, which prosecutors use to take up defamation cases and national security violations. The increasing frequency of such lawsuits, the report said, risks a “chilling effect on the right to freedom of expression.”

In Song’s case, the restrictions on his Twitter account only brought him more attention. The KCSC has the authority to block domestic Web surfers, but it cannot block access through smartphones, so Song can tweet with his iPhone. He has more than 23,000 followers, many of whom access his tweets via cellphone. These days, he says, he targets more of his criticism at the KCSC than the president.

“I am trying to enjoy the fight,” he said.

Special correspondent Yoonjung Seo contributed to this report.