Before Zhou Enlai's death he had been involved in a power struggle with others in the politburo, the infamous Gang of Four led by Jiang Qing, Mao Zedong's wife. Mourning for Zhou was limited by directives of the Communist Party of China, the flag, for example, being lowered to half-staff for only one hour.
April 4 is celebrated in China as Qing Ming, the Festival for Tending Graves. Prior to Qing Ming mourners had begun placing paper wreaths at the Monument to the People's Heroes in Tiananmen Square as well as white paper chrysanthemums. On April 4 there was an outpouring of wreaths at the Monument as hundreds of thousands of Beijingers flocked to the square. Of particular interest were many handwritten poems that were posted there. These poems on their surface appeared to a naive observer such as Jan Wong a young overseas Chinese student from Canada, to commemorate ancient events from Chinese history, but, in fact, they expressed critical sentiments regarding those in power in China, for example one poem attacked the Empress Wu Zetian a 7th century Tang Dynasty empress who ruled after her husband died, an allusion to Jiang Qing.
Such a massive outpouring of sentiment alarmed the government who meeting in emergency session as the Politburo in the Great Hall of the People just west of the Square determined to remove all the wreaths and poems. This was done that night.
The next day tens of thousands of citizens returned to the Monument in Tiananmen Square and were dismayed to find the wreaths and other materials removed and a police cordon preventing approach to the Monument. Things became boisterous and inside the Great Hall of the People China's rulers were alarmed. After consultation with Mao it was determined to use force to clear the square. Mao authorized the use of force but not guns.
That evening when only a few thousand protesters remained in the square they were driven from the square by militia armed with clubs. Four thousand were arrested. Sixty protesters were dragged into the Great Hall of the People, beheaded and later shipped to Shanghai and secretly cremated.
See also: Tiananmen Square protests of 1989