The date on which troops shot their way into central Beijing in 1989, killing hundreds, was not publicly marked in mainland China. The democracy protests in Tiananmen Square and elsewhere remain taboo for the ruling Communist Party, especially this year after calls for an Arab-style "jasmine revolution."
In Hong Kong, tens of thousands lit candles, held jasmine flowers and chanted for a fully democratic China in a night vigil to mark the anniversary and condemn Beijing's human rights abuses and curbs on freedoms.
The State Department said China must release all those still jailed for their participation in the 1989 protests.
"We ask the Chinese government to provide the fullest possible public accounting of those killed, detained or missing," deputy spokesman Mark Toner said.
At least five people remain in jail for taking part in the protests.
Chinese Foreign Ministry spokesman Hong Lei, in a statement carried by the official Xinhua news agency, said the U.S. comments "groundlessly accused the Chinese government."
"We urge the U.S. side to abandon its political bias and rectify wrong practices to avoid disturbing China-U.S. relations."
The president of democratic Taiwan, the island China claims as its own and has never renounced the use of force to recover, said Beijing should follow Taipei's example and reform politically.
"We urgently hope the mainland Chinese authorities will have the courage to undertake political reforms and promote the development of freedom, democracy, human rights, and rule of law," President Ma Ying-jeou said in a statement.
On Saturday, Tiananmen Square was packed with tourists as normal, with no obvious signs of extra security.
"I didn't agree with the method of the protest, making a disturbance on the square," said a 60-year-old Beijing resident who gave her family name as Chen. "But I think there should be a way for people to express what's on their mind."
"VOICE FOR CHINA"
In a Hong Kong park however, some 150,000 people made a plea for Beijing to atone for the June 4 crackdown, an event given added poignancy this year by a heavy clampdown on dissent.
"We want to express that we've never given up," said Andy Wong, who was at the vigil with his wife and two kids. "When there's a big turnout it shows that we (Hong Kong) still care."
Hong Kong, a former British colony handed back to China in 1997 with a promise of a high degree of autonomy, has remained a beacon for the overseas Chinese pro-democracy movement.
"Hong Kong is now playing a more important role when the whole of China is silenced," said Lee Cheuk-yan, a pro-democracy lawmaker and one of the organizers. "We are the voice for China and we'll spread the message for democracy,"
Dissidents in China, meanwhile, said controls over them had been strengthened.
"I can't come out today. I've been kept at home. But I'll be fasting for the day, like I do every June 4 anniversary," said Zhou Duo.
Zhou was one of four activists who negotiated with troops to evacuate Tiananmen Square of student-protesters in 1989, avoiding much bloodshed on the square itself on June 4. He was later jailed for his role in the protests.
"Of course, sooner or later June 4 will be reassessed and rehabilitated. That's inevitable. History can never be completely erased."
Zhang Xianling, who lost her son in the Tiananmen protests, said she had been allowed out to visit her son's grave, but was being followed and was not allowed to go as part of a group with other bereaved parents, as she has done in the past.
"It shows that even after all these years, China is still limiting human rights," Zhang said.
After the crackdown, the government called the movement a "counter-revolutionary plot," but has more recently referred to it as a "political disturbance."
Recent unrest in Inner Mongolia and explosions in two provinces sparked by social grievances have also ruffled authorities as the leadership prepares to hand over power to a new generation at a Party Congress next year.