China on Wednesday imposed smothering security in central Beijing on the 25th anniversary of the Tiananmen Square crackdown, as tens of thousands gathered in Hong Kong for potentially the biggest commemoration yet seen in honour of the student protesters killed in 1989.

The White House called for China's Communist authorities to account for those killed, detained or missing in connection with the June 1989 assault, which remains a taboo topic for a nation that refuses to allow political reform in line with its dramatic economic transformation.

Evening candlelit vigils were due to take place in Macau and Taipei as well as Hong Kong, where organisers said a record crowd as large as 200,000 people would attend a vast gathering in a downtown park.

Taiwanese President Ma Ying-jeou, whose democratically elected government still proclaims itself to be the true representative of China, described the events of 25 years ago as an "enormous historical wound".

Ma called on Beijing to "speedily redress the wrongs to ensure that such a tragedy will never happen again".

Likewise, the United States will continue to "urge the Chinese government to guarantee the universal rights and fundamental freedoms that are the birthright of all Chinese citizens", a White House statement said.

Hundreds of unarmed civilians -- by some estimates, more than 1,000 -- were killed during the June 3-4 crackdown of 1989, when soldiers on foot and in tanks crushed months of peaceful protests by students who were demanding political liberties to match China's nascent economic opening up.

Thousands of police and other security forces, some armed with automatic rifles, have been deployed across the Chinese capital this week. There were numerous police trucks on and around Tiananmen Square Wednesday, with fire engines and ambulances also visible.

Some security officers had fire extinguishers placed ready nearby. Security has also been heightened recently after a spate of attacks that authorities blame on separatists from the far-western region of Xinjiang.

Tourists and vendors criss-crossed the vast public square in the heart of the city, but uniformed and plainclothes officers were stationed at every corner and checking the ID cards of passers-by.

An AFP journalist was ordered to delete photos of scuffles between police and frustrated pedestrians waiting to enter the main part of the square Wednesday morning.

- 'Serious consequences' -

In 1989 the demonstrations and subsequent crackdown played out on television screens across the world, and Beijing briefly became a pariah in the West.

But 25 years later, the ruling Communist Party's authority is intact and its global clout continues to rise in line with the country's rapid growth, which has taken China to second place in the global economic rankings behind only the United States.

UN human rights chief Navi Pillay on Tuesday called on China to "finally establish the facts surrounding the Tiananmen Square incidents".

Hong Lei, a Chinese foreign ministry spokesman, condemned both Pillay's statement and earlier remarks by the US government as interference in China's "internal affairs".

Since 1989, China has worked hard to erase public memories of the bloodshed, censoring any mention of the incident from online social networks and detaining scores of activists, lawyers, artists and relatives of victims in recent weeks.

Among those detained ahead of this year's anniversary is prominent human rights lawyer Pu Zhiqiang, who along with four others was taken away by authorities last month after attending a private seminar discussing the crackdown.

Around 80 academics from 12 countries -- including some of the world's foremost China experts -- penned an open letter to President Xi Jinping on Wednesday pressing for the release of the five.

Many foreign news outlets have received warnings from police and the foreign ministry against newsgathering related to the anniversary -- or risk "serious consequences" including the possible revocation of their visas.

- Social networks scrubbed -

Under pressure from authorities, Chinese online social networks quickly deleted any perceived references to the crackdown, banning terms including "Tiananmen", "student movement", "6/4" and "25th anniversary".

A handful of mentions managed to slip past the censors, however, including one posting that showed an image of a candle and the date June 4, 1989.

Asked about the anniversary, university students declined to be interviewed or suggested they did not know much about it.

"I know about it but I don't really understand what it is," said a student near Peking University. "At this time, we were not even born!" she added.

The state-run Global Times on Wednesday made references to the anniversary in an editorial in its English-language edition, but the Chinese-language version only alluded to it.

"Chinese society has never forgotten the incident 25 years ago but not talking about it indicates the attitude of society," the English-language editorial said.