Open discussion of the crackdown is forbidden in China, where hundreds -- by some estimates more than a thousand -- died when the Communist Party sent tanks to crush demonstrations in the square in Beijing on June 4 1989, after student-led protesters had staged a peaceful seven-week sit-in to demand democratic reforms.

In an open letter to Xi dated "the eve of 2018 June 4th", the Tiananmen Mothers, an association of parents who lost children in the violence, said: "each year when we would commemorate our loved ones, we are all monitored, put under surveillance, or forced to travel".

AAP

"No one from the successive governments over the past 29 years has ever asked after us, and not one word of apology has been spoken from anyone, as if the massacre that shocked the world never happened," said the letter, which was released on Thursday by the non-profit Human Rights in China.

"The 1989 June Fourth bloody massacre is a crime the state committed against the people. Therefore, it is necessary to re-evaluate the June Fourth massacre," the letter said, calling for "truth, compensation, and accountability" from the government.

The protests are branded a "counter-revolutionary rebellion" by Chinese authorities and many on the mainland remain unaware of the crackdown, with discussion banned from books, textbooks, movies and censored on social networks.

The semi-autonomous territory of Hong Kong is the only place on Chinese soil where the anniversary is openly marked with a famous vigil in Victoria Park on June 4 each year.