Tens of thousands of people have held a candlelight vigil in Hong Kong to mark the bloody crackdown on pro-democracy protesters 25 years ago in Beijing's Tiananmen Square, while mainland China sought to censor any mention of the 1989 event.

Demonstrators holding candles and clad mainly in black gathered in a downtown park in Hong Kong on Wednesday and called on Beijing to atone for the killings.

Organisers said about 180,000 people took part of the event in the former British colony, which returned to Chinese rule in 1997 but remains a free-wheeling, capitalist hub.

This event must be instilled in everyone's heart. We can't let time dilute this. Anna Lau, Hong Kong student

People bowed to pay their respects as footage of the clampdown was shown on large screens.

"This event must be instilled in everyone's heart. We can't let time dilute this," said 19-year-old student Anna Lau.

A large number of mainland Chinese also flocked to commemorate the crackdown in the city-state territory, where a vigil has been held every year since the massacre.

"Hong Kong is a free society where you can speak out. In China, the Communist Party dictates everything," said Chen Jing Gen, in his 60s, who travelled from the southern Chinese city of Shenzhen to attend the vigil.

"People in mainland China are mostly aware of June 4, but due to the control of the Party no one dares to talk about it," Chen added.

Al Jazeera's Rob McBride, reporting from Hong Kong, said: "It is an annual event here to keep alive symbolically the flame of democracy that was lit 25 years ago. People here think they have a bond with those students who came out in Tiananmen Square all those years ago."

Beijing on alert

In Beijing, police flooded the streets around the square, scene of the worst of the violence a quarter of a century ago, and censors scrubbed the internet clean of any mention of the rare open display of defiance against the Communist Party.

Dozens of riot police and police patrol cars could be seen parked around the square, as well as at intersections several blocks away on the avenue of Eternal Peace.

"This time 25 years ago today you could not enter Tiananmen Square as a journalist. And 25 years on you still can't," said Al Jazeera's Adrian Brown, reporting from Beijing.

"This really is a reflection of the official nervousness for this very sensitive anniversary. They are trying to erase all public memory of what happened here 25 years ago."

Despite the date's sensitivity, operations at the square seemed to resume as they would on a normal day in Beijing.

'Internal affairs'

The United States led international calls for China to account for what happened on June 4, 1989.

The comments riled China, which has said the protest movement was "counter-revolutionary".

"We demand the US side respect China's judicial sovereignty and not make irresponsible comments on issues that are related to China's internal affairs," Chinese Foreign Ministry spokesman Hong Lei told a regular news conference in Beijing.

Exiled Tibetan spiritual leader the Dalai Lama also used the anniversary to call on China to embrace democracy.

China has never released a death toll from the crackdown after troops shot their way into central Beijing, but estimates from human rights groups and witnesses range from several hundred to several thousand.

China also prohibits public discussion of the events, leaving many of the country's youth ignorant of what happened.

"I had never heard of the Tiananmen incident until I was studying in the United States when I was 18," said a 25-year-old woman surnamed Lan, who was visiting Hong Kong from Beijing.

Activists and commentators in the mainland said the country's determination to quell any mention of the event was especially strong this year, and authorities had made mass arrests just before the anniversary.

Rights group Amnesty International said 48 people had been detained, placed under house arrest, questioned by police or had gone missing ahead of the anniversary.

Police have also warned foreign reporters from visiting "sensitive places" and prevented them from interviewing people on the topic.