High security on Tiananmen Square
Many dissidents say they have been told to leave Beijing or are confined to their homes.
Xiang Xiaoji, now a US citizen, was trying to come to Hong Kong on Wednesday to join commemorative events being held to mark the anniversary. But he was refused entry and returned to New York.
Another prominent protest leader, Wu'er Kaixi, flew from his home in Taiwan to Macau on Wednesday, but was detained on arrival.
On the eve of the anniversary, police have been examining visitors at checkpoints dotted around Tiananmen Square, and checking the bags and papers of people in the area.
Some journalists say they have been turned away from the site.
Ding Zilin, the head of a group called Tiananmen Mothers - made up of women whose children were shot dead in the crackdown - has reportedly been blocked from leaving her home, as has the wife of jailed dissident Hu Jia.
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