Censors at China's top Twitter-like microblogging site blocked searches for "big yellow duck" and removed the candle icon used to express mourning as part of the country's annual June 4 David-and-Goliath face-off pitting Chinese Internet users eager to memorialize the 1989 crackdown on Tiananmen Square protesters against China's sprawling online censorship apparatus.

With the arrival of the anniversary on Tuesday, Sina Corp.'s Weibo appeared to have rethought a brief experiment with more sophisticated filtering. Late last week, previously blocked searches on the site for terms related to the crackdown suddenly began to produce innocuous, carefully selected results. But beginning Monday night, those searches went back to being blocked.

Sina's censors were also busy hand-deleting Tiananmen-related images from the site, including a series of tongue-in-cheek re-imaginings of the iconic "Tank Man" photo, taken as Chinese military vehicles were attempting to leave the square.

In one version, produced by cartoonist Hexie Farm, a blue bird from the Rovio game "Angry Birds" stands before a line of porcine tanks, middle feather raised in a defiant gesture. Other users posted various recreation of the Tank Man tableau using Legos.

Among the most striking: An image that replaced the tanks with a line of oversized rubber ducks – a reference to the giant yellow duck currently occupying Hong Kong's Victoria Harbor that has sparked a flood of duck-related merchandise and cheap imitations on the mainland.