BEIJING - Chinese security forces blanketed Tiananmen Square on Wednesday ahead of the 20th anniversary of the June 4 crackdown on pro-democracy protesters, and a day after Twitter and other Internet services in China were blocked.

Black police vans lurked at the side of the Forbidden City, near the square, while police and paramilitary forces patrolled through crowds of tourists enjoying a sunny summer morning.

Tanks rolled into Tiananmen Square before dawn on June 4, 1989 to crush weeks of student and worker protests. The ruling Communist Party, which has never released a death toll, fears any commemoration of the crackdown could challenge its continuing hold on power.

Business is poor today. You'd think most people are tourists but they aren't, they are plainclothes security, said a trinket peddler surnamed Li, before a plainclothes policeman told her to stop talking to foreigners.

They are scared there will be a big blow-up because of tomorrow, but I don't think anything will happen.

The government has also sought to stifle any risks of protest by confining dissidents. Among them has been Yu Jie, a writer in Beijing, who said he can only leave his home on the city outskirts for short outings accompanied by two police officers.

Yu said the tight security showed that memories of 20 years ago remained politically volatile.

I don't think June 4 has been forgotten, but under pressure of fear, people are afraid to speak out, he said.

OUTWARD CALM

Compared to 10 years ago, however, the outward calm around Tiananmen suggested the government is now more sure of its grip on power and on public sentiment, with the population generally better off.

Around the tenth anniversary of the crackdown, the square was closed to the public for months, with the government saying it needed repairs.

About 30 people are still serving prison sentences for their activities in 1989, according to the San Francisco-based human rights organization Dui Hua. Others are in prison for continued activism after their initial release.

Hundreds more protest leaders are in permanent exile.

Wu'er Kaixi, one of the best known, tried to reach China via Macau on Wednesday but was sent back, Taiwan authorities said.

Wu'er Kaixi, 41, who now lives in Taiwan, said the airport immigration counter in Macau stopped him from entering the special administrative region of China.

He said he had planned to apply for a visa in Macau to enter the Chinese mainland and visit his parents.

China on Tuesday blocked access to popular Internet services Twitter and online photo sharing service Flickr, as well as briefly interrupting email provider Hotmail. Foreign newscasts about the anniversary were also cut.

Police prevented at least four foreign television crews from filming on the square in the week before the anniversary.

Administrators at Chinese universities have been told to keep a close eye on foreigners in their departments. Taxi drivers were instructed to watch out for suspicious passengers, especially those headed toward the square.

The campus of elite Peking University, an incubator of protest in 1989, was quiet on Wednesday morning. Students were preoccupied with exams rather than politics.

Xia Yeliang, an economics professor at the university, has been one of the small group of people openly urging the government to denounce the killings in 1989.

He has called on the Internet for citizens to wear white, the traditional color of mourning, on June 4.

Students these days are more concerned about getting good grades and good jobs, said Xia. They really don't known much about 1989, but they do know it's too sensitive to talk about.

(Additional reporting by Benjamin Kang Lim and Chris Buckley; Editing by David Fox)

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