In February–March 1948, it instigated general strikes in opposition to the plans to create a separate South Korean state.

[4] On 3 April 1948, the party led a popular uprising on Jeju island, against the unilateral declaration of the foundation of South Korea.

In the suppression of the revolt, thousands of islanders were killed, largely by forces of the South Korean Government.

[citation needed] In one of its first official acts, the South Korean National Assembly passed the National Security Act in September 1948, which among other measures, outlawed the Workers' Party of South Korea.

In the Korean War, 60,000 to 200,000 members of the party and suspected communist supporters, many of them civilians, were massacred by the South Korean Army with supervision of the US army[7] in what became known as the Bodo League massacre.