Chinese authorities have tightened security around Tiananmen Square in Beijing as Saturday marked the 33rd anniversary of the military crackdown on pro-democracy demonstrators.

On June 4, 1989, Chinese troops opened fire on students and others who gathered in and around the square to demand democracy. The incident resulted in a large number of casualties.

The Chinese authorities maintain that they made the right decision in responding to the protests. In a historical resolution adopted last year, the Chinese Communist Party referred to the incident as "turmoil".

Publicly discussing the incident has been regarded as taboo in China, where information is strictly controlled.

On Saturday, a large number of police officers were mobilized around the square and makeshift checkpoints were set up to check visitors' belongings.

The number of visitors to the square is restricted until June 15 as part of the city's anti-coronavirus measures.

A group of relatives of the victims called the "Tiananmen Mothers" posted a letter on the internet addressed to China's government and its leaders.

The letter says that the unilateral, fact-denying characterization and rhetoric of China's ruling party and government are devoid of humanity. It says that they will not withstand the test of history.

A core member of the group, Zhang Xianling, lost her 19-year-old son in the crackdown.

The 84-year-old mother said she would be convinced that her son didn't die in vain only when the incident is thoroughly investigated and light is shed on the truth.

The Chinese government and Communist Party are apparently trying to suppress any criticism against them in the run-up to a party congress scheduled later this year. At the congress, it is thought that President Xi Jinping will seek an unprecedented third term as the party's leader.