Bloody May Day (血のメーデー事件, Chi no mēdē jiken) refers to a violent conflict that took place between protesters and police officers in the Kokyo Gaien National Garden in front of the Imperial Palace in Tokyo, Japan, on May 1, 1952.

[2] After Japan was defeated in World War II, a United States-led military occupation ruled the country for seven years, from 1945 to 1952.

Three days after the coming into force of these treaties, on May 1—a traditional date for annual "May Day" protests in socialist and leftist circles—the left leaning national labor federation Sōhyō made plans for a nationwide day of protest in cities and towns across Japan to convey widespread popular outrage at the one-sided peace and security treaties that would enshrine Japan's "subordinate independence" under U.S. hegemony, and the Japanese government's failure to secure the retrocession of Okinawa.

[1] Meanwhile in 1950, at the behest of Soviet premier Joseph Stalin, the Soviet-led Cominform had published a tract harshly criticizing the policies of the Japanese Communist Party (JCP) of peaceful protest and engagement with electoral politics, which led the JCP to completely change its policies and attempt to foment an immediate, violent revolution in Japan along Maoist lines.

[4] In May 1952, the JCP was still attempting to foment a violent communist revolution, and thus, supported by radical student activists from the nationwide Zengakuren student federation and Zainichi Korean activist groups, the JCP sought to infiltrate Sōhyō's peaceful May 1 protest movement and instigate the masses to engage in violent attacks on police and American military targets.

Although most gatherings around the nation were peaceful, violence broke out in central Tokyo when protesters attempted to occupy the plaza in front of the Imperial Palace (present-day Kokyo Gaien park).

[1] In the fall 1952 general election, Japanese voters punished the Communist Party for their violent policies, and the JCP lost every single one of its 35 seats in the lower house of the Diet.