Why did Liu Shaoqi work underground in Shanghai during the early 1930s?

During the early 1930s, Liu Shaoqi remained in Shanghai working underground because he was actively involved in movements opposing the Japanese invasion of Manchuria and their subsequent encroachment into North China.1

Although the Chinese Communist Party had largely shifted its focus from urban centers to rural areas by 1931, Liu continued his clandestine activities in the city to agitate against these foreign incursions. He maintained this underground role until late 1932, when he eventually joined Mao Zedong’s forces in Jiangxi province.1