Several governments including the US urged China to account for what happened on June 4, 1989, comments that riled China, which has said the protest movement was "counter-revolutionary".
Exiled Tibetan spiritual leader the Dalai Lama used the anniversary to call on China to embrace democracy.
China has never released a death toll for the crackdown, but estimates from human rights groups and witnesses range from several hundred to several thousand.
Troops shot their way into central Beijing after demonstrators had clogged Tiananmen Square in Beijing for about six weeks. There were also protests in many other cities.
Yesterday police, soldiers and plainclothes security personnel enveloped Tiananmen Square, checking identity cards and rummaging through bags looking for any hint people might try and sneak onto the square to commemorate the day.
Public discussion of the crackdown is off-limits in China and many young people are unaware of what happened 25 years ago.