China's state censors on Thursday scrubbed the Internet of references to commemorations of the Tiananmen crackdown including a huge vigil in Hong Kong, extending a campaign of repression that has seen dozens of critics detained.

Organisers said a record 180,000 people filled Hong Kong's Victoria Park for Wednesday night's gathering, the only major commemoration on Chinese territory of the 25th anniversary of the events of June 4, 1989.

The assault on pro-democracy protesters in Beijing, during which hundreds of unarmed civilians -- by some estimates, more than 1,000 -- were killed, remains highly sensitive in the Communist nation.

It forbids public discussion of the military's brutal suppression of the demonstrations, and dozens of individuals, among them human rights campaigners, lawyers and journalists were detained ahead of the anniversary.

Five, including celebrated human rights lawyer Pu Zhiqiang, were held on criminal charges last month after taking part in a private seminar about the crackdown.

Two of the group -- writer Hu Shigen and academic Xu Youyu -- were released on bail Thursday afternoon, dissident Hu Jia said, citing associates. A third detainee -- writer Liu Di -- has also been released, her father said.

In mainland China's state-run media there were few references to the events of 1989 and none to the Hong Kong candlelight vigil which made world headlines, with images of the southern city's Victoria Park turned into a sea of twinkling lights.

China also hit back at a call from the US for it to account for those killed, detained or missing in the crackdown, accusing Washington of blaming its government "for no reason".

Internet censorship was already tightened ahead of the anniversary, with searches for the date "6.4" and similar terms blocked by Sina Weibo, a Chinese alternative to Twitter, and on popular search engines.

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

Following the vigil, the list of blocked search terms expanded to include "Victoria Park", "candlelight", and "Teng Biao", the name of a leading Chinese human rights lawyer who delivered a blistering critique of Beijing in the park.

Users who attempted to search for any of the blocked terms were greeted with a message explaining that results were not displayed "in accordance with relevant laws, regulations and policies".

Hong Kong has held an annual remembrance of the Tiananmen crackdown since 1989, and Wednesday's vigil was the largest ever, according to organisers. Police put the turnout at 99,500 -- nearly twice as many as their estimate last year.

Under the agreement governing Hong Kong's handover to China in 1997, the semi-autonomous city has far greater civil liberties than the mainland.

- Official silence -

China's official Xinhua news agency has made only three oblique references to the "June 4 incident" -- Beijing's euphemism for the crackdown -- to condemn US, UN and Japanese calls to improve human rights and release activists detained ahead of the anniversary.

"Twenty-five years ago, the United States deplored the use of violence to silence the voices of the peaceful demonstrators in and around Tiananmen Square," a White House statement said.

"Twenty-five years later, the United States continues to honour the memories of those who gave their lives in and around Tiananmen Square and throughout China, and we call on Chinese authorities to account for those killed, detained or missing in connection with the events surrounding June 4, 1989."

Chinese foreign ministry spokesman Hong Lei rejected the US criticism, which comes at a time of growing distrust between the world's two largest economies, and accused it of interfering in Chinese domestic politics.

At a regular briefing with reporters on Thursday, he again defended Beijing's handling of the crackdown.

"On the political upheaval in Beijing, the Chinese government long ago reached its conclusion," Hong said. "In the last 30 years of reform and opening up, we have made remarkable economic progress."

On Wednesday in Shanghai, China's commercial capital, a news scroll on the government-run metro system under the headline "Today in History" gave no hint of the significance of June 4 to China.

One of the events listed was: "1989 -- Poland holds its first democratic elections".