Two key defendants were sentenced to death today under South Korea's anti-communist national security laws for their roles in the March l8 arson attack on the American Cultural Center in the country's southern port city of Pusan.

The Pusan district court's action was interpreted by observers in Seoul as a sign that the government of President Chun Doo Hwan intends to deal harshly with opponents of his two-year-old rule.

The case, involving the setting of a fire in which one Korean student was killed, came at the end of a trial that has been the focus of anti-government protest in South Korea in recent months.

Sentenced to death were student leaders Kim Hyon Jang, 31, and Mun Bu Shik, 23. Under South Korean law, sentences involving capital punishment are automatically reviewed by higher courts.

Two female former college students were given life sentences, and eight other persons were ordered to serve jail terms ranging from two to 15 years. Choi Ki Shik, a Roman Catholic priest, was sentenced to three years in prison for harboring fugitives involved in the case.

The arson attack, with its anti-American focus, outraged much of the South Korean public and the country's strongly pro-U.S. establishment. Diplomatic sources in Seoul interpreted it as an isolated act of violence by a handful of non-communist radicals.

But many leaders of the country's large Christian minority, while deploring the violent act, have suggested that it was symbolic of a mounting anti-American mood among political activists in South Korea who perceive Washington as a key source of support for the Chun government.

Dissident churchmen have become increasingly critical of what they view as the Reagan administration's desultory approach to human rights and have suggested more anti-American activity can be expected.

Prosecutors charged that Kim and Mun instigated the incident to promote "socialist reforms." They argued that anti-governnment leaflets scattered near the scene of the fire, calling for the withdrawal of the 39,000 U.S. troops stationed in the country, indicated an attempt to foment a "grassroots revolution" in line with Communist North Korean propaganda.

According to defense lawyers, the defendants had no communist links and set the fire to protest U.S. policy toward South Korea. "Contemptuous remarks" regarding Koreans allegedly made by U.S. Ambassador Richard L. Walker and former U.S. commander Gen. John A. Wickham, Jr. also prompted the attack, they said. Walker and Wickham have denied making such statements.