IT MAY seem like a fairly harmless term, but 'Big Yellow Duck' has the Chinese authorities in a flap after a mocked-up picture of the famous tank protest in Tiananmen Square circulated online.

The picture shows plastic ducks in the place of military vehicles in the 1989 "Tank Man" photograph, where a civilian stares down a long row of tanks headed toward Tiananmen Square.

The image could not be found on domestic websites within the "Great Firewall of China", as the country's system of internet control is known.

Authorities launch a major push every June 4 to prevent discussion of the violent 1989 pro-democracy protests, China's darkest human rights stain in recent decades, in which hundreds of people died.

High-profile dissident Hu Jia wrote on Twitter the broad crackdown on discussion of Tiananmen demonstrated the government's weakness.

The event underscored the ruling Communist Party's "illegal nature", he wrote. "The unprecedentedly high pressure for the 24th anniversary of June 4 actually reveals the authorities' fear."

The ban on the popular duck comes as Chinese police are blocking the gate of a cemetery housing victims of the Tiananmen crackdown on its 24th anniversary, part of a sweeping annual effort to bar commemorations of the event.

More than a dozen security officials were deployed outside the stone gate at the Wanan graveyard near the hills of western Beijing yesterday, where mothers of the victims visit each year.

And in a narrow street close to Beijing's Forbidden City, security personnel patrolled outside the former house of Zhao Ziyang, the former communist party secretary who was purged and held under house arrest following the protests.

Authorities have also detained or enhanced surveillance of 10 prominent dissidents, according to the Hong Kong-based advocacy group China Human Rights Defenders.

Online searches for a wide range of keywords on Sina Weibo, China's version of Twitter, were blocked, from "Tiananmen" to "candle", which was used to encourage digital vigils.

Activists turned instead to overseas websites to commemorate the event and criticise authorities.

"The dispute in this country is basically stuck on whether to light a candle or to extinguish it," dissident artist Ai Weiwei posted on Twitter.

Rights lawyer Liu Xiaoyuan said on Twitter that he had been blocked from Sina Weibo for seven days for sharing "sensitive information" - urging others to honour victims by posting an image of a lit candle.

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