Human rights and pro-democracy groups say hundreds, possibly thousands, were killed when soldiers moved into crush the protests on June 4.
China's government has never officially acknowledged what happened during the crackdown.
Phelim Kine, a Hong Kong-based researcher for Human Rights Watch, told Al Jazeera it is difficult to know the precise number of Tiananmen prisoners still in Chinese jails since the issue remains taboo in China.
"There are prisoners' lists that have been compiled by relatives of these people who are known to be in prison," he said.
"These people were not political counter revolutionaries which is what they have been charged with by the Chinese government. These were students, these were mechanics, these were writers."
The US government has also marked the 19th anniversary of the Tiananmen crackdown with a call for Beijing to give a full account of people killed, missing or detained.
"The time for the Chinese government to provide the fullest possible public accounting of the thousands killed, detained or missing in the massacre that followed the protests is long overdue"
US state department
"The time for the Chinese government to provide the fullest possible public accounting of the thousands killed, detained or missing in the massacre that followed the protests is long overdue," the US state department said in a statement.
China has not provided any official figures for those still in prison for their connection with the demonstrations in which hundreds, possibly thousands, were killed.
Liu Xiaobo, an internet writer who was jailed for nearly five years after the protests, said he knows of up to eight people serving life sentences in a Beijing prison for their role in the protests.
The square itself was reported calm on Wednesday, with the usual crowds attending the daily flag raising ceremony at dawn.
For many Chinese the 1989 crackdown is now a fading memory – while many are too young to recall it.
One former senior Chinese official has however spoken out to urge the government to be more transparent in its accounting of the crackdown.
Bao Tong, a former top aide to purged Communist party chief Zhao Ziyang, said that China should adopt the same approach it has shown in handling the last month's deadly Sichuan earthquake to the events in Beijing 19 years ago.
"Through this quake ... they have tasted the benefits of openness and should know that openness is better than being closed," Bao told Reuters in an interview at his Beijing home.
"June 4 of 19 years ago was a man-made disaster, but like natural disasters it should be made known to the people of the entire country and the whole world," he said.
Bao's former boss, Zhao Ziyang, was ousted as party chief in 1989 for opposing the decision to send in the military to crush the Tiananmen protests.