SEOUL, AUG. 27 -- The South Korean government announced a crackdown today on "radical leftist forces" that it said have infiltrated hundreds of strikebound factories and college campuses in a plot to overthrow President Chun Doo Hwan's regime before presidential elections are held later this year.

"We are now standing at a grave crossroads," Prime Minister Kim Chung Yul said in announcing the government's renewed hard-line stand against dissidents. He spoke after an emergency Cabinet meeting on the eve of a planned protest for a laborer killed Saturday by a police tear-gas grenade.

"The voice of seditious, subversive forces is growing, while that of sound conservative groups is diminishing," the prime minister said. "The leftist forces will not be allowed to find a toehold in this society."

Justice Minister Chung Hae Chang said that he was preparing charges against 60 subversives he said were arrested while inciting workers to take part in the illegal strikes and slowdowns that have paralyzed key South Korean industries in recent weeks.

Kim warned that if the disorder continued it would jeopardize Chun's policy of leniency, which was announced June 29 amid anti-government rioting.

Several South Korean political analysts said they viewed the government's return to the sort of hard-line rhetoric it used before the June 29 announcement as a warning to student leaders who plan to join the labor protests after they return to the universities on Sept. 1.