Media playback is unsupported on your device Media caption As Juliana Liu reports, there were also protesters who disagreed with the vigil

Tens of thousands have gathered in Hong Kong for the only major commemoration in China of the 25th anniversary of the Tiananmen Square massacre in Beijing.

The organisers said some 180,000 attended the vigil, but the police put the crowd size at just under 100,000.

The city retains civil liberties not permitted to mainland Chinese.

The 1989 protesters wanted political reform, but the crackdown was ordered after hardliners won a power struggle within the ruling Communist Party.

In Beijing, the authorities have imposed blanket security, particularly on Tiananmen Square, to prevent any attempts to mark the anniversary.

Dozens of activists were detained in the run-up to the anniversary, with foreign journalists ushered away from the square on Wednesday.

Lights out

The Chinese authorities classify the 1989 protests as counter-revolutionary riots and hold no memorial.

But in Hong Kong a large crowd joined the Tiananmen remembrance rally, which has been held every year since the massacre.

At the scene: Juliana Liu, Hong Kong correspondent

Hong Kong is a special Chinese city with a high level of autonomy, where civil liberties like freedom of speech and assembly are guaranteed. Here, people from all walks of life were allowed to gather openly to mourn the violent suppression of a peaceful protest in Beijing in 1989.

It was a solemn, emotional evening filled with music and video appearances from well-known Chinese dissidents. They called on the Chinese government to apologise for the crackdown, to come clean on exactly what happened and on how many people were killed, and to commit themselves to democratic reforms.

Image copyright Reuters Image caption The candles turned Hong Kong's Victoria Park into a sea of flickering light

Image copyright AFP Image caption Participants wore white headbands with the writing "4 June 25 years"

With the lights turned out, people raised candles in the dark as the names of those who died in Beijing on 4 June 1989 were read over loudspeakers.

"Let (Chinese President) Xi Jinping see the lights of the candles," rally organiser Lee Cheuk-Yan was quoted by the AFP news agency as saying.

Activist groups in Taiwan also marked the anniversary. Taiwanese President Ma Ying-jeou described China's crackdown as "an enormous historical wound".

Both the Taiwanese and Japanese governments urged Beijing to use the memory of the protests to improve its attitude to human rights.

Vietnam has broken its long silence on the massacre. State media were highly critical of the Chinese government in a sign of the growing tension between the two countries over territorial disputes in the South China Sea.

Tiananmen protests

From 1978, China opened up its economy to the world, but communists maintained total control over politics

In 1989, hundreds of thousands gathered in Beijing's Tiananmen Square to call for political reform

Protesters remained in the square for weeks while a power struggle raged within the ruling Communist Party

Hardliners prevailed and gave the order to remove the protesters by force; hundreds were massacred in nearby streets

Timeline: Tiananmen protests

Why the protests still matter

Chinese media quiet on anniversary

In pictures: Tiananmen protests

Struggling against Tiananmen censors

In Tiananmen Square itself, passers-by were searched and had their papers checked.

Some of the relatives of those killed in the massacre were allowed to visit the graves of their loved ones under police guard.

Rights group Amnesty International said in a statement that 66 people had been detained, questioned or had gone missing.

Image copyright AFP Image caption Paramilitary police joined regular forces to control Tiananmen Square, in the centre of Beijing

Image copyright AP Image caption But Chinese tourists thronged the square as usual, and seemed to notice little out of the ordinary

The US on Wednesday called on the Chinese authorities "to account for those killed, detained or missing in connection with events surrounding 4 June 1989".

Internet search terms related to the 1989 massacre and the protests have been blocked in China, and access on Google has reportedly been restricted.