SEOUL >> In late 2014, months after 304 people died in the sinking of a South Korean ferry, a leaflet began circulating with a scurrilous rumor about President Park Geun-hye: that she had failed to respond swiftly to the disaster that day because she was having a romantic encounter with a former aide.

Was Park, the flier asked, now cracking down on her critics in an attempt to keep that scandal from coming to light?

For Park Sung-su, an anti-government campaigner who had distributed the leaflet — and who is not related to the president (Park is a common surname here) — the consequences soon followed. He was arrested and later sentenced to a year in prison on charges of defaming the president and staging illegal protests against his prosecutors. He was freed in December after eight months, when a court suspended his sentence.

No evidence supporting the rumor has been produced, and prosecutors said they had investigated and found it groundless. But however dubious the leaflet might have been, opponents of the government say Park became another victim of the very thing he was denouncing: the government’s use of defamation and other laws to silence its critics, which rights advocates say is on the rise.

Last year, the United Nations Human Rights Committee warned against South Korea’s “increasing use of criminal defamation laws to prosecute persons who criticize government action.” Freedom House, a rights group based in Washington, criticized “the increased intimidation of political opponents” under Park, who took office in 2013.

“The government is especially sensitive about defending the personal reputation of the president,” said Park Kyung-sin, a professor of law at Korea University who has researched the issue.

The constitution guarantees freedom of expression. But defamation laws here carry penalties that include prison — up to three years for comments that are true and up to seven for statements considered false — if they are deemed not in the public interest. Critics say the distinction is vague and opens the door to abuse by prosecutors.

Charges that her administration was suppressing political rights sprang up almost as soon as Park — whose father, the military dictator Park Chung-hee, ruled South Korea with an iron fist during the 1960s and ’70s — assumed office.

In 2013, the National Intelligence Service, a powerful spy agency that her father used to torture and silence dissidents, moved to disband an outspoken progressive party. The agency arrested the party’s leaders on charges of violating the Cold War-era National Security Law, which bans activities deemed pro-North Korean.

Distrust of the spy agency deepened after it was accused of interfering on Park’s behalf during the 2012 presidential campaign. A former director of the agency, Won Sei-hoon, was convicted of running a team of officers who posted comments online criticizing Park’s rivals before the election. In 2014, agency employees were convicted of fabricating Chinese immigration documents to concoct a spy case against an ethnic Chinese refugee from North Korea.

When human rights lawyers and journalists brought those tactics to light in 2013, saying the agency was resorting to old habits of coercion and faking evidence, counterintelligence officials responded by filing defamation cases.

The government’s use of the defamation laws became a diplomatic issue when Japan complained after the 2014 indictment of Tatsuya Kato, a journalist from the Japanese newspaper Sankei Shimbun, for reporting the rumor about Park and her former aide.

An opposition lawmaker, Park Jie-won, said the indictment had embarrassed the country. “It’s prosecutors who should be indicted for defaming South Korea,” he said.

Kato was acquitted in December. But Park, the lawmaker, has been charged with defamation after airing suspicions that the former aide and others close to the president had arranged for favored officials to be promoted. Presidential aides sued six journalists from a South Korean newspaper for reporting similar allegations, which Park’s office denied.

Besides the defamation laws, the government’s use of the National Security Law has long been seen by international human rights groups, as well as the U.S. State Department, as a threat to free speech. Since 2014, a Chinese student and a Korean-American lecturer have been deported for comments seen as sympathetic to North Korea. Longstanding fears of the North, especially among conservatives, have stymied efforts to repeal or revise the law.

The government’s policing of the Internet, a popular channel for anti-government grievances, was already an issue under Park. In 2014, after months of withering criticism about how she had handled the ferry disaster, Park warned that some of it had gone “too far.” Prosecutors soon announced a crackdown on “false or defamatory data in cyberspace.”

Kakao Talk, a popular smartphone-based messenger service, then admitted that it had been cooperating with the police and prosecutors to collect the online chat records of thousands of users, including anti-government demonstrators. Last month, a court ruled in favor of one of those protesters, concluding that the inspection of her records was unlawful because it had been conducted without her knowledge.

Such rulings aside, critics argue that South Korean prosecutors and judges have largely failed to protect the public’s rights, often because they want to earn the favor of politicians who can promote them.

“People are lamenting that there are no watchdogs, but only dogs,” Kwon Seok-cheon, a columnist for the newspaper JoongAng Ilbo, recently wrote.

Park Sung-su, the activist who spread the rumor about the president and her former aide, made use of the same metaphor after police officers raided his home. He called them “running dogs for the government,” later throwing dog food at the gates of police stations. In April, he was arrested on the charge of staging an illegal rally and then interrogated after he and several other activists had shouted, “Bow wow!” in front of a prosecutors’ office.

“They kept asking me what was the political meaning of ‘bow wow,’” he said.

© 2016 The New York Times Company