Neither prosecutors nor the intelligence service revealed details of the treason charges against the opposition politicians. The national news agency Yonhap, quoting unnamed intelligence officials, reported that they were accused of plotting to sabotage communications, oil facilities and other installations as part of a plot to overthrow the South Korean government, a charge the progressive party called absurd.

Like many other members of his party, Mr. Lee, the lawmaker, is a former student activist who was prosecuted under the country’s anti-Communist national security laws. He served a prison sentence for participating in an underground political party that was manipulated by the North Korean government during the 1990s.

Since he and other progressives won seats in the National Assembly in 2012, some conservative South Koreans have attacked them as “jongbuk,” or blind followers of North Korea. The progressive party’s platform calls for “rectifying our nation’s shameful history tainted by imperialist invasions, the national divide, military dictatorship, the tyranny and plunder of transnational monopoly capital and chaebol,” the latter referring to South Korea’s giant family-controlled business conglomerates. The party wants to end the American military presence, dismantle South Korea’s “subordinate alliance with the United States” and unify the North and the South. In a television interview last year, Mr. Lee said that “a problem far bigger than jongbuk” was blindly following the United States, or “jongmi.”

Conservatives have often accused progressives here of being too quick to question their country’s alliance with Washington but too reluctant to say a harsh word about North Korea over human rights abuses and the pursuit of nuclear weapons. Before she was elected president, Ms. Park once proposed a parliamentary vote to force Mr. Lee from the legislature, calling his ideology “questionable.”

Treason charges were sometimes used by South Korea’s former military dictators to arrest dissidents, but after the country was democratized, the tables were turned: two former presidents, Chun Doo-hwan and Roh Tae-woo, were convicted in 1996 of mutiny and treason for their roles in a 1979 military coup and 1980 crackdowns on a pro-democracy uprising in the southern city of Kwangju that left hundreds killed.

On Wednesday, the United Progressive party said that the raid was reminiscent of the Yushin, or “revitalization,” era, when Ms. Park’s father, Park Chung-hee, ruled the country with an iron fist. He came to power in a military coup in 1961 and ruled for 18 years; during his tenure, dissidents were tortured and sometimes executed without a proper trial on the same kinds of accusations now leveled at Mr. Lee. The National Intelligence Service, once known as KCIA, was a favorite tool in campaigns to frame the dictators’ political opponents as North Korea sympathizers; successive governments since then have vowed to reform the agency and keep it out of domestic politics.