In Tiananmen Square, police were ready to pounce at the first sign of protest. In Hong Kong, a sea of candles flickered in the hands of tens of thousands who vented their grief and anger. Two starkly contrasting faces of China were on display Thursday, the 20th anniversary of the military's bloody crackdown on pro-democracy demonstrators - from Beijing's rigid control in suppressing any dissent, to freewheeling Hong Kong, which enjoys freedoms all but absent on the mainland. Tiananmen Square was blanketed by uniformed and plainclothes security officers who were ready to silence any potential demonstration, and there were few hints that the vast plaza was the epicenter of a student-led movement that was crushed on June 3-4, 1989, shocking the world. Police barred foreign journalists from entering the square and threatened them with violence, even barring them from covering the daily raising of China's national flag. Chinese and foreign tourists were allowed in Tiananmen as usual, although security officials appeared to outnumber visitors. Dissidents and families of victims were confined to their homes or forced to leave Beijing, part of sweeping government efforts to prevent online debate or organized commemorations of the anniversary. But in Hong Kong's Victoria Park, a crowd chanted slogans calling for Beijing to own up to the crackdown and release political dissidents. Organizers estimated its size at 150,000, while police put the number at 62,800. "It is the dream of all Chinese people to have democracy!" the throng sang. Hong Kong is one of the few places in China where the events of June 1989 are not off-limits, because the territory - returned by the British 12 years ago - operates under a separate political system that promises freedom of speech and other Western-style civil liberties. "Hong Kong is China's conscience," Hong Kong pro-democracy lawmaker Cheung Man Kwong told the demonstration. In the candlelight, speakers recalled the terrifying events in Tiananmen, where a military assault killed hundreds who had gathered for weeks in the square to demonstrate for freedom and even erect a makeshift statue of liberty. Those killed were eulogized as heroes in the struggle for a democratic China, their names read aloud before the crowd observed a minute of silence. "Hong Kong is the only place where we can commemorate, and we have to repeat this every year so our younger generations don't forget," said Annie Chu, 36, a Hong Kong tourism worker who says she has attended every vigil for the last 20 years. Earlier in the day, the central government ignored calls from US Secretary of State Hillary Rodham Clinton and even Taiwan's China-friendly president for Beijing to face up to the 1989 violence. The extraordinary security in Beijing came after government censors shut down social networking and image-sharing Web sites such as Twitter and Flickr and blacked out CNN and other foreign news channels each time they showed stories about Tiananmen. "We've been under 24-hour surveillance for a week and aren't able to leave home to mourn. It's totally inhuman," said Xu Jue, whose son was 22 when he was shot in the chest by soldiers and bled to death on June 4, 1989. Police were also stationed outside the home of Wang Yannan, the daughter of Zhao Ziyang, the Communist Party leader deposed for sympathizing with the pro-democracy protesters, according to the Hong Kong-based Information Center for Human Rights and Democracy. Wang has never been politically active. But Zhou was celebrated in Hong Kong. Tape recordings of Zhou recalling Tiananmen, used for his recently released posthumous memoir, were played over loudspeakers next to his portrait. One former student leader, Xiong Yan, stirred the crowd with predictions that "democracy will arrive in China." Another student leader from 1989, Wu'er Kaixi, was forced to return to Taiwan on Thursday after flying to the Chinese territory of Macau the day before in an attempt to return home. In Washington, Clinton said Wednesday that China, as an emerging global power, "should examine openly the darker events of its past and provide a public accounting of those killed, detained or missing, both to learn and to heal." Taiwanese President Ma Ying-jeou urged China to lift the taboo on discussing the crackdown. "This painful chapter in history must be faced. Pretending it never happened is not an option," Ma said in a statement. Chinese Foreign Ministry spokesman Qin Gang attacked Clinton's comments as a "gross interference in China's internal affairs." Qin refused to comment on the security measures - or even acknowledge them. "Today is like any other day, stable," he said. Beijing has never allowed an independent investigation into the crushing of the protests in 1989, in which possibly thousands of students, activists and ordinary citizens were killed. In one famous moment of resistance, a lone man holding shopping bags defiantly stood in front of a column of tanks on a street near the square. Young mainland Chinese know little about the events, having grown up in a generation that has largely eschewed politics in favor of raw nationalism, wealth acquisition and individual pursuits. But the issue still resonates with Hong Kong's younger generations. "It's time for China to take responsibility for the killings," said Kin Cheung, a 17-year-old Hong Kong student who attended the yearly vigil for the first time Thursday. "They need to tell the truth."