He is world famous, and at the same time anonymous. The man who stood alone blocking a column of tanks at Tiananmen Square endures as a symbol of peaceful protest and defiance 25 years later.

It was just before noon on June 5, 1989. Wearing a white shirt, carrying a shopping bag in each hand, he strode out a day after Chinese troops killed hundreds of pro-democracy protesters in the heart of Beijing.

In the middle of the wide avenue running north of Tiananmen Square -- by now empty of the students who had dreamed of democracy -- he stopped, facing the first of a column of tanks and armoured vehicles stretching far down the road.

Captured on camera, "Tank Man" has become one of the defining images of the 20th century. Unforgettably powerful, his photograph has been endlessly reproduced, despite being censored at home by China's ruling Communist Party.

His identity and fate are unknown.

The first tank repeatedly tried to move around him. Each time, he stepped back into its path.

"I felt he was saying: 'I won't let you pass, back off -- we're prepared to die here,'" said Hu Jia, a prominent dissident in Beijing.

"What he did symbolises the spirit of young people at that time."

The protester climbed onto the vehicle to engage in a conversation with one of the tank crew, as gunshots crackled in the air.

Climbing down, he stood to one side, gesticulating for the column to go back down the road. But when the lead tank tried to speed past him, he dashed back into position to stop it again.

Eventually, he was pulled away by two men. By some accounts they were security agents; others said they were worried onlookers.

- Flustered leader -

The stand-off lasted only minutes. But Tank Man's calm and courage have earned him a place in history -- his mystique reinforced by his disappearance, probably at the hands of the security forces.

For a quarter of a century theories have swirled about him, but few facts have emerged. Some identified him as Wang Weilin, a name that has never been confirmed. Nor has that of the tank driver who refused to crush him.

Like many others, Hu Jia tried to find out who the protester was -- and even asked a friend in the military to try to track down the soldier at the wheel -- but without success.

From Chinese authorities, there has been nothing but a wall of silence.

A year after the crackdown, American TV journalist Barbara Walters confronted Jiang Zemin, by then number one in the Communist Party, with a photograph of Tank Man.

"Do you have any idea what happened to this young man?" she asked.

A flustered Jiang stressed that the tanks had not run the man over. He declined to confirm the man's fate beyond saying that he did not believe he had been killed.

- 'Unknown Soldier' -

Several photographers captured Tank Man's lone figure on film that day. But it was Jeff Widener of the Associated Press news agency whose shot became the most widespread, nominated for a Pulitzer Prize and now regarded as one of the most recognisable photographs of all time.

Taken from the balcony of his room at the Beijing Hotel, it was splashed across the front pages of newspapers the world over -- except, of course, in China.

Since 1989 it has been recycled countless times -- by rights groups denouncing repression, in advertising campaigns, and by satirists. A parody version appeared in "The Simpsons", with Homer Simpson refusing to budge before a column of taxis.

But in China the image remains largely unrecognised, heavily censored by Communist authorities. Hu Jia, for instance, did not see it until several years after it became world-famous.

"Not many people owned cameras in those days, so the image is incredibly valuable," said Hu.

Last year, a satirical version of the shot -- with giant yellow rubber ducks replacing the tanks -- went viral online. China responded by banning the phrase "big yellow duck" from Internet searches.

Widener, now 57, confesses that he developed a "love-hate relationship" with his photo in the years that followed, as it overshadowed all of his later work.

"I think about Tank Man from time to time and wonder what happened to him," Widener, who now lives in the German city of Hamburg, told AFP.

"Perhaps it is better we never know who he is," he added. "It's a bit like the 'Unknown Soldier'. He will always remind us of the importance of freedom and democracy and our rights for human dignity."

Hu agrees. "Maybe he was killed, maybe he was thrown in prison, maybe he went abroad, but it doesn't matter any more," he said.

"I believe we are all Tank Men. If we confront the system, Tank Man lives on."