In 1957, Mao Zedong launched the Anti-Rightist Campaign, which strove to attack Chinese democracy adherents, academics, and intellectuals. More than 500,000 people were tragically affected.
In February 1956, Nikita Khrushchev, General Secretary of the Communist Party of the Soviet Union, held an interim, secret meeting after the 20th National Congress. At the meeting, Khrushchev slammed former General Secretary Joseph Stalin, saying that he had created great terror for the country; Khrushchev encouraged a return of power to the Politburo. The "De-Stalinization" of the Soviet Union caused huge turmoil in the communist world. Revolts broke out in Hungary, Poland, and other places, and petition demonstrations were held across China. Following this wave of revolt, Liu Shaoqi, Deng Xiaoping, Zhou Enlai, and others began to advocate for collective leadership, and expressed opposition to personality cults. As a result, Mao Zedong felt deeply threatened.
On April 25, 1956, Mao gave a speech titled "On the Ten Major Relationships" to an expanded Politburo meeting, as a response to the criticism. He said that China should no longer follow Stalin's old path, but should embark on its own socialist path. He encouraged intellectuals to express their views freely; as he put it, they should "let a hundred flowers bloom and a hundred schools of thought contend." In November of that year, the Soviet Union sent troops to suppress the Hungarian rebellion. Mao criticized the Hungarian Socialist Workers' Party (Magyar Szocialista Munkáspárt, MSZMP) for failing to solve the problems of the masses in time, saying the MSZMP had become a "aristocracy separated from its people". Mao decided to launch a political campaign within the party, while continuing to call on the masses to help in rectifying the Chinese Communist Party (CCP)'s politics, and to be courageous in giving criticism.
By May 1957, the wave of criticism against the CCP had turned into a storm. Some criticized corruption at the top of the CCP, the Party's heavy-handed control, and even demanded the implementation of democracy and human rights. Comments from members of the China Democratic League (one of the PRC's recognized political parties), including Chu Anping, Zhang Bojun, Luo Longji and others, were particularly sharp, saying that the CCP had come to view itself as the "entire world".
On May 15, 1957, Mao delivered an article called "Things Are Beginning to Change" to leaders within the CCP. In it, he said that when the time was right, he would strike back. However, to the outside world, he was still advocating for people to "speak loudly, speak broadly". Beginning in June of that year, the number of articles refuting "rightists" in the "People's Daily" began to gradually increase; the "Anti-Rightist Campaign" began to develop. Mao handed responsibility over the movement to Deng Xiaoping; hundreds of thousands of people were attacked in the Anti-Rightist Campaign.
Many intellectuals began to expose each other; many were criticized at a conference held in the Great Hall of the People. They were even physically abused right there, and there were no few instances of suicide. Chu Anping, who had once published the "CCP as the entire world" speech, disappeared shortly after being persecuted and was pronounced dead. Zhang Bojun and Luo Longji were labeled as the "Zhang-Luo Anti-Party Alliance". Since Mao's criteria for what constituted a rightist was extremely vague, anyone who had expressed an opinion could be labelled a rightist. Some were labelled rightists just for inspiring envy in others; one person was sent to a concentration camp for praising American-made shoe polish, an act which was seen as "blindly worshiping foreign imperialist things".
References: Frank Dikötter, "The Tragedy of Liberation: A History of the Chinese Revolution, 1945—1957"; 《中華人民共和國史（第三卷）：思考與選擇－從知識分子會議到反右派運動（1956-1957）》("History of the People's Republic of China, vol. 3: Thought and Choice — From the Conference on the Issue of Intellectuals to the Anti-Rightist Movement (1956-1957)") (Chinese)