If there had not been a Tiananmen crackdown 25 years ago, Mo says he probably would have never embarked on a career as a human rights lawyer.

Mo, 56, is one of China’s longest serving and most prominent rights lawyers, having represented political activists, journalists and dissidents for nearly two decades.

Speaking recently about how he started defending government critics, he said it was his sympathy for people implicated for their links with the Tiananmen pro-democracy movement that spurred him to defend them.

In the 1990s, only veteran lawyer Zhang Sizhi and Mo would take on political cases, many of which involved former Tiananmen activists, he said.

"In those days, no other lawyers dared represent them. These people were deprived of their rights to be defended [in court],” he said.

“We were very isolated,” Mo said. “At the time, when you took up sensitive cases, the judiciary bureau would summon you and told you to uphold the party and the government’s image and [you] must give face to the court.”

He said he took up his first “sensitive” case in 1995, when the wife of veteran democracy advocate Liu Nianchun could not find other lawyers to defend him.

Liu was sentenced to three years of “re-education through labour” for helping organise a signature campaign calling for democracy and the rule of law upon the sixth anniversary of the crackdown on the Tiananmen pro-democracy movement.

Since then, he has defended former university lecturer Liu Xiaobo, who supported students on Tiananmen Square and later launched a signature campaign to urge the government to reassess the event. Liu was jailed three times before being sentenced again in 2009 to 11 years in prison on the charge of “inciting subversion” for co-authoring the Charter 08 democracy manifesto.

Mao’s other clients have included dissidents Jiang Qisheng and Zhang Lin, political activists Xu Wenli, Lu Gengsong and Zhu Yufu and writer Du Daobin.

Mo, who spent four years in the army as a young man, said he used to harbour “good feelings” towards the Communist Party and joined it in 1979. But the Tiananmen crackdown in 1989 shocked him.

"I thought the students did nothing wrong. How could the army open fire and use tanks on ordinary people?” he said.

The incident convinced him that one-party rule must be changed.

"If that doesn’t change, there won’t be genuine democracy and the rule of law,” he said.

"[The crackdown] strengthened my belief that a country should embark on the road of rule of law and constitutional democracy. It’s the best system to safeguard people’s freedoms and basic human rights … and makes a country genuinely prosperous,” he said.

Mo said the government must confront its mistake and disclose the historical facts about the June 4 crackdown. He said that would be a pretext to reconciliation and the elimination of animosity between the government and the people.

"As a starting point, you must disclose the truth. If you don’t tell people the truth, you will never gain the people’s understanding,” Mo said, adding that facts such as how many people were killed and which leaders ordered the troops to open fire have never been revealed.

"And if there is no reconciliation, there will be hatred forever, and the country has no future,” he said.

The life of Cao, 68, a leading liberal intellectual, was also completely changed by the Tiananmen incident. But 25 years on, his ideals still remained strong.

Throughout most of the 1980s, he worked in government think tanks researching economic reform issues, famously proposing and drafting the mainland’s first bankruptcy law. A year before the crackdown, he stepped down to join a private think tank to focus on pushing the authorities to take on political reform.

During the Tiananmen pro-democracy movement, Cao was one of the intellectuals asked by the government to persuade students to stop their hunger strikes and to return to their classes.

And days before the crackdown, Cao garnered signatures from scores of members of the National People’s Congress Standing Committee to urge the parliament to hold an emergency session to peacefully resolve the stand-off on Tiananmen Square. The move failed to stop the crackdown and the authorities investigated those who were involved.

For these efforts, Cao was accused of being a “black hand” who masterminded the pro-democracy movement, and he was arrested in the afternoon of June 3, just hours before troops descended on Beijing to suppress protesters.

Cao was secretly detained for nearly a year in Beijing’s Qincheng Prison, during which time he was repeatedly interrogated about his relationship with student demonstrators. The police did not immediately inform his family, and for months, they presumed he had died in the crackdown.

"There was no charge, no trial, no sentencing. It was illegal detention,” he reminisced recently. The authorities could not find evidence against him but didn’t want to release him either, he said.

Twenty five years later, Cao is still agitated by the failure of his efforts to prevent the tragedy.

"[The crackdown] was completely preventable – if the National People’s Congress could hold such a meeting, there would have been no need to open fire,” Cao said. “I tried my best to prevent this disaster from happening, but they didn’t take my advice.”

Cao blamed the country’s constitution for touting “the dictatorship of the proletariat”, which, in Marxist doctrine, justifies the use of “ruthless, extra-judicial oppression” against reactionary forces in the struggle between the proletariat and the bourgeoisie. The resulting lawlessness meant no one could be spared, he said.

"The manifestation of this was the troops opening fire. It was the suppression of the enemy class,” he said. “This [concept] is the root of all evils.”

He said the government must reassess its earlier verdict that Tiananmen was “counter-revolutionary unrest”, but what should be an even more urgent and workable task is for a new clause to be introduced in the constitution to bar the army or police from opening fire on civilians.

"To prevent the tragedy from happening again, can we depend on the leaders?” he asked. “We have to incorporate it into the system: the army and police cannot open fire on ordinary people.”

He said this suggestion should be the least difficult option for top leaders to accept in regards to addressing “the June 4 issue”.

He nonetheless insisted that the government must reverse the verdict on Tiananmen, saying reconciliation with Tiananmen victims and ordinary people could only happen when it admits its fault.

"When the verdict is reversed, then the Communist Party can lay down its burden,” Cao said, pointing out that after the party vindicated Cultural Revolution victims in the early 1980s, the country was able to carry out reforms and make progress.

"If the June 4 issue is not resolved, the country cannot progress,” Cao said.