"I urge the Chinese authorities to immediately release those detained for the exercise of their human right to freedom of expression," Pillay said in a statement.
Dozens of individuals, among them human rights campaigners, lawyers and journalists are believed to have been detained as Wednesday's anniversary of the June 4th 1989, crackdown looms.
Several were reportedly charged with "creating a disturbance" because they took part in a private discussion about the events of 1989, which remain highly sensitive for China's communist regime.
China forbids public discussion of what happened in 1989 when the military brutally suppressed pro-democracy protesters, mainly students, who had staged a mass sit-in on Tiananmen Square in central Beijing.
Troops killed hundreds of unarmed civilians — by some estimates more than 1,000.
Pillay noted that Chinese authorities had also slapped anniversary-related restrictions on social media, traditional media and internet users.
"Rather than stifle attempts to commemorate the 1989 events, the authorities should encourage and facilitate dialogue and discussion as a means of overcoming the legacy of the past," she said.
She also said it was crucial for China to come to terms with what happened 25 years ago.
"In the absence of an independent, factual investigation, there are dramatically differing accounts," Pillay said.
"The death toll, for example, ranges from hundreds to thousands, and many families of victims are still awaiting an explanation of what happened to their loved ones," she said.
"It is in the interests of everyone to finally establish the facts surrounding the Tiananmen Square incidents."
Pillay underlined that China had made strides since 1989, particularly in the area of economic and social rights, as well as legal reforms.
"Learning from events of the past will not diminish the gains of the past 25 years, but will show how far China has come in ensuring that human rights are respected and protected," she said.