For some years now, the discussion of the 1989 clampdown when the Chinese government set tanks and troops on peaceful protestors is all but forbidden in mainland China.

Tiananmen Square Massacre: What really happened in 1989?

Searches for the famous Tiananmen Square 'Tank Man' protest photo came up empty on Microsoft search engine Bing on Friday, raising censorship concerns on the anniversary of the deadly crackdown.

Famed 'Tank Man' photo vanishes from Bing search engine

Here is a timeline of developments since China imposed national security legislation in Hong Kong last year, making anything Beijing regards as subversion, secession, terrorism or colluding with foreign forces punishable by up to life in prison.

Timeline: The impact of the national security law on Hong Kong

Hong Kong police detained prominent democracy activist Chow Hang-tung on Friday morning, with authorities determined to prevent any protests on the anniversary of Beijing's deadly Tiananmen crackdown.

Hong Kong police detain democracy leader on Tiananmen anniversary