Tens of thousands of people in Hong Kong on Saturday marked the bloody 1989 Tiananmen Square crackdown, as China defies world condemnation with an ongoing roundup of political dissidents.

A sea of people, mostly clad in black as a sign of mourning, held up candles and sang solemn songs -- some with tears in their eyes -- filling the city's Victoria Park, in the only commemoration on Chinese soil.

Police said 77,000 people had crammed into the park, an area the size of the six football pitches, but organisers put the crowd at more than 150,000.

"I am here with a heavy heart, it is very emotional for me," Gladys Liu, a 48-year-old mother-of-two told AFP.

"I still remember the scenes -- how the army tanks were sent in to break up the student-led protests. I was following the news closely, I never thought it would turn so violent," said Liu, who brought her son, 14, and nine-year-old daughter to the vigil.

"I want my children to know what happened. This is not something that can be learned in school."

Hundreds, perhaps thousands, are believed to have died when the government sent in tanks and soldiers to clear Beijing's Tiananmen Square on the night of June 3-4, 1989, bringing a violent end to six weeks of pro-democracy protests. An official verdict after the protests called them a "counter-revolutionary rebellion" although the wording has since been softened.

"I want to know the truth of June 4," said 17-year-old student Melissa Tang, who was among many youngsters holding a candle aloft at the vigil.

"Maybe we don't have the full picture of what happened but the students at the 1989 protests deserve our respect."

Wang Dan, a key leader of the 1989 protests, and Ding Zilin, spokeswoman of the Tiananmen Mothers, a group representing families of the victims, addressed the crowd through pre-recorded video messages from Taiwan and China respectively.

The massive crowd also stood for a minute's silence to pay tribute to those who died.

Organisers condemned the crackdown as "the darkest day in modern Chinese history".

"We are sending the strongest message to the (Chinese) communist regime that they cannot suppress the memory of June 4," said Lee Cheuk-yan, chairman of the Alliance in Support of Patriotic Democratic Movements of China which organised the event.

"They should start political reform and return the power to the people," said Lee, also a prominent lawmaker, as he urged China to immediately release all political detainees.

Hong Kong, a former British colony, returned to Chinese rule in 1997 but retains a semi-autonomous status with civil liberties -- including the right to protest -- not enjoyed in mainland China.

In Beijing, where the Tiananmen protests remain a taboo topic, thousands of Chinese and foreign tourists flocked to the giant square on Saturday amid a heavy police presence, but many shied away from answering questions on the incident.

China attempts to block any public discussion or remembrance of the events by hiding away key dissidents in the run-up to June 4 each year, taking them into custody or placing them under house arrest, friends and activists say. Rights groups including New York-based Human Rights Watch have repeated calls for China to be held accountable for its past and present actions, but Beijing reiterated its position that the matter was closed.

"As for the political turbulence that took place in the last century in the late 1980s, the Communist Party and government have already made a conclusion," foreign ministry spokesman Hong Lei said Thursday.

Since mid-February, as protests spread across the Arab world leading to the toppling of leaders in Tunisia and Egypt, Chinese authorities have detained dozens of lawyers, activists and dissidents in an ongoing clampdown.

The vigil comes after Chinese police for the first time raised the possibility of compensation for those killed, according to Tiananmen Mothers. The group said in an annual open letter this week that police have twice met relatives of one victim beginning in February, in a possible sign that Beijing is changing its view on the June 4 crackdown.

The letter said, however, that police did not discuss a formal apology for the killings or a public account of who ordered the shootings -- two of the group's long-standing demands.