In the late 1950s, Chongryon conducted a campaign to persuade Zainichi Koreans to migrate to North Korea. The campaign was vehemently opposed by Mindan which organised hunger strikes and train obstructions. Some 87,000 Zainichi Koreans and about 6000 Japanese spouses moved to the North. According to a defector, himself a former returnee, many petitioned to be returned to Japan and in response were sent to political prison camps. Japanese research puts the number of Zainichi Korean returnees condemned to prison camps at around 10,000.