Microsoft has blamed "accidental human error" for its search engine Bing failing to produce any results for 'Tank Man' on the anniversary of the Tiananmen Square protests.

Users in the UK, US, Germany and Singapore were met with the message "There are no results for tank man" when they searched for it on Friday - the 32nd anniversary of the demonstrations.

'Tank Man' refers to an image of a lone protester standing in the path of an oncoming tank during the suppression of student-led protests in Beijing's Tiananmen Square in June 1989.

Although the man was never identified, it has become the defining image of the military crackdown, which many refer to as the Tiananmen Square Massacre, and killed between hundreds and thousands of people.

Microsoft has been accused of censoring the image and co-operating with China, which refers to the suppression as the 'June 4 incident' and bans any discussion of it.

Although China applies censorship to search engines operating in its jurisdiction, it has no power to ban content beyond its borders.

A large number of Microsoft employees who work on Bing are located in China.

Tiananmen Gate in Beijing is pictured on 4 June.

Kenneth Roth, of Human Rights Watch, tweeted to say the "inadvertent error" was "hard to believe". "Outrageous," he posted.

David Greene, civil liberties director at the Electronic Frontier Foundation said that content moderation often poses challenges for search engines and "egregious mistakes are made all the time".

But he added: "At worst, this was a purposeful suppression at the request of a powerful state."

Protesters in Hong Kong still gathered despite restrictions.

In Hong Kong, where China passed national security laws earlier this year, protests were less busy than usual after authorities banned them. People wore masks and held up their smartphones in lieu of the usual candlelit vigil.

The organiser barrister Chow Hang Tung, 36, was arrested hours before it took place.

Microsoft said in a statement that the issue was "due to an accidental human error" and the site was "actively working to resolve this".

The 'Tank Man' images had returned to the search engine in the UK early on Saturday.