A popular Chinese microblogging service banned searches for "yellow duck" after users circulated a mocked-up image of a famous 1989 Tiananmen square tank protest with the military vehicles replaced by plastic ducks, results on Wednesday showed.

The picture, a parody of the iconic "Tank Man" photograph of a civilian staring down a long row of tanks, circulated Tuesday, the 24th anniversary of the Tiananmen protests' suppression.

A large yellow duck artwork is currently on display in Hong Kong, and imitations have been put up in several mainland cities.

Sina Weibo, China's most popular microblogging site, blocked searches for "yellow duck", with attempts to do so returning a message reading: "Due to relevant laws and regulations... the search result is not displayed."

Other search terms banned by the service include "1989" and numerical formulae such as "63 + 1", used to refer to the date of the crackdown -- "6.4" in US-style date order, which is also blocked.

Discussion of the Tiananmen crackdown, in which at least several hundred people died, is strictly controlled by China's ruling Communist party, which is wary of challenges to its authority.

Reports say that Sina, the company which runs Sina Weibo, employs teams of censors to delete posts, generally exercising self-censorship rather than receiving direct instructions from the Party.