With her usual police escort in tow, Ding Zilin went out yesterday morning to buy six white and four red roses. She planned to lay them at the spot where soldiers shot her son exactly 20 years ago, but did not even get close.

Hundreds of police patrolled the subway entrance in western Beijing where the retired philosophy professor was trying to commemorate the death of Jiang Jielian, shot through the heart shortly before midnight on June 3, a day after he turned 17.

He was one of the first victims of the crackdown as the Communist Party brought extra troops into the capital to stifle seven weeks of pro-democracy demonstrations centred in Tiananmen Square.

As yesterday’s 20th anniversary of the massacre approached, the Communist Party was determined to prevent a commemoration of what it calls a “counter-revolutionary” uprising. Anyone expected to mourn the hundreds of people killed by the troops on the night of June 3-4 was placed under tight surveillance or house arrest.

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At noon yesterday, when tanks started moving along the Changan Avenue, I watched as a man in a white shirt ran from a crowd

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Tiananmen Square had been cleared by nightfall. Thousands of police, many in vans with their red and blue lights flashing, patrolled its vast circumference. It was a show of force intended to deter even the most courageous.

Ms Ding, 72, the founder of the Tiananmen Mothers’ group campaigning for a true account of the bloody events 20 years ago, said in the morning that nothing would stop her. Just to buy the roses had been a battle. She told The Times: “The police wouldn’t let me out of my house. I told them I would die before they could stop me. I said they could follow me.”

She planned to place the flowers on the spot at Muxidi on the broad Avenue of Eternal Peace in western Beijing, about three kilometres from Tiananmen Square, where her son died.

That is where the first bursts of gunfire rang out 20 years ago. The demonstrators had not expected soldiers to use live ammunition. Few had taken cover. Most thought at first that the troops were firing rubber bullets. But as protesters fell and blood flowed the reality dawned on them.

Filled with panic, residents raced away from Muxidi, dragging with them the injured on flat-bed tricycles normally used for transporting cabbages. “Go back, go back,” they shouted to anyone trying to reach Muxidi. They pushed others back to protect them from the hail of bullets raining down on the streets.

Ms Ding, along with the mothers and relatives of other victims, has battled for years for an explanation of what happened after the People’s Liberation Army entered Beijing. But her first chance to commemorate her only child did not come until 2007 when, at last, the police let her go to the spot where he was killed. She took flowers and candles and splashed wine on the street.

Last year Ms Ding was again able to pay her respects. But last night the moment passed. She did not appear and all contact with her had been lost, her home telephone ringing unanswered.

Most likely Ms Ding, like other activists in China, had been detained by the police. She would not have been surprised. As she said earlier: “Now I see that in 2007 and 2008 they were just putting on a show for the Olympics. Now that the United States needs China’s money to buy its treasuries they have set human rights to one side. I am absolutely furious.”

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