They have tightened security around Tiananmen Square on Friday by posting vans at entrances and checking the identification of visitors.
Thousands of tourists had already gathered on the square, following the squawking speakers of their flag-carrying guides while sightseeing on the symbolic political heart of China, a Reuters correspondent said.
Hundreds, perhaps thousands, were killed on the night of 3-4 June 1989 when People's Liberation Army soldiers backed by tanks shot their way through intersections blocked by Beijing residents and wrested control of the square.
Police have gagged leading pro-democracy activists by putting them under house arrest or forcing them out of the capital in the run-up to the politically sensitive anniversary, activist groups say.
A lone man staged a short-lived protest on the square on Thursday night and was swiftly taken away by police, a witness said.
A lone man staged a short-lived protest on the square on Thursday night and was taken in by police, a witness said
The man, about 50, kneeled briefly to pray at the foot of the Monument to the Peoples' Heroes at the centre of the square, where tens of thousands of students gathered from April to June 1989 to press demands for democratic reform.
Police in plain clothes and uniform routinely comb the square on sensitive anniversaries, snuffing out protests as quickly as they start.
The man's identity and cause could not immediately be determined. There were no further details.
"The Chinese government is trying to wipe out the memory of Tiananmen Square, but the horror of what happened still resonates inside and outside China," said Kenneth Roth, executive director of the New York-based Human Rights Watch.
Hong Kong, where calls for more democracy have been growing, plans a candlelight vigil at Victoria Park on Friday evening.
"Police are now outside my door. They said if I do not agree not to light candles to remember June 4 victims, they will take me away. In mainland China, to survive is absurd," Hu Jia, a 30-year-old Chinese AIDS activist, said in a text message on Thursday evening. He could not be reached on Friday.
London-based rights group Amnesty International called for an independent inquiry into "the killing of unarmed students and demonstrators" in the protests and for the government to release those who never had fair trials.
But analysts said rehabilitation of the protests was unforeseeable in the near future because such a move would be too politically sensitive.