Thousands of tourists flooded into Beijing's Tiananmen Square yesterday, the 21st anniversary of the Chinese army's deadly crackdown on mass democracy protests, but the police presence was largely muted.

Unlike last year, which marked the 20th anniversary of the crackdown on the night of June 3-4, when foreign reporters were banned from the square and nervous authorities blanketed the area with security, the police presence yesterday was low key.

Although there were only dozens of uniformed police officers with earpieces scattered across the giant square, a few hundred security volunteers, each wearing white T-shirts with a Beijing Olympic Games logo, could be seen all over the square.

Vans belonging to the city police patrolled the square and black cars marked "special police" - each manned with two armed officers wearing helmets and flak jackets - were seen at regular intervals in the city centre.

While the masses of tourists pouring through checkpoints to visit the square shied away from responding to questions about the anniversary, apparently because of the sensitivity of the issue, there were some exceptions.

Environmental activist Su Yutong was picked up by police and plainclothes officers when she tried to lay flowers at the Monument to the People's Heroes in the middle of the square. She was questioned for nearly six hours and then escorted home.

"I was just sitting near the monument, holding a bunch of white roses when I was spotted by some volunteers," she said. "I was soon surrounded by police and plainclothes officers who asked me why I had taken flowers to the square. They pushed me into a van and took me to their office.

"I told them all I wanted to do was know more about the truth of that chapter of history and observe a moment of remembrance," she said.

"The government is still trying to cover it up and list it as taboo for public discussion. I feel sad that fewer and fewer people know about the crackdown," said Su, 30.

Her brief custody caused a stir on Twitter yesterday, with many users expressing concerns over her safety and anger towards authorities' attempts to clamp down on any commemorative activity marking the sensitive anniversary.

China attempts to block any public discussion or remembrance of the events by hiding key dissidents in the run-up to June 4 each year, taking them into custody or placing them under house arrest, friends and activists say.

They include Qi Zhiyong, who lost a leg in the crackdown, and rights defender Yang Qiuyu.

According to the Hong Kong-based Information Centre for Human Rights and Democracy, two former student activists, Ma Shaofang and Guo Haifeng, have been missing since Thursday. They had planned to come to Hong Kong to take part in the Victoria Park vigil.

Families of jailed activists Liu Xiaobo and Zhou Yongjun openly called on the government to grant medical parole for them, said the rights group.

But some internet users managed to get past the nation's vast censorship system - dubbed the "Great Firewall of China" - to commemorate the event. Most comments were promptly deleted.

On popular Web portal sina.com, one blogger posted an image of a black square with a pool of blood, adorned with the sentence: "We will not forget!"

Agence France-Presse

and Staff Reporter