1989 Mao portrait vandalism incident
During the Tiananmen Square protests of 1989, the portrait of Mao Zedong at Tiananmen was defaced. At 2:00 pm, May 23, 1989, three young protesters from Liuyang, Hunan, posted banners on the wall of the Tiananmen gate's passway.
Source: Wikipedia — 1989 Mao portrait vandalism incident (CC BY-SA 4.0)