"Press freedom should have been the first step in political reform, but there is still no law on it … I guess it will never come out," Hu Dehua said.

Watch: 25 years on, China still remembers former Party leader Hu Yaobang

"When you have no law to protect these rights, everything is in the hands of the officials.

"Although we have a constitution which guarantees freedoms in speech and assembly… in fact, there are hardly any freedoms. We have no right to supervise [the government]."

He added: "Today, corruption among officials is impossible to rein in and ethnic tension is intense. There was a window for reform then [in the 1980s] but it was missed and I don't know when the next one will come."

Hu Yaobang's name has for years been taboo and mostly censored by official media.

Over the weekend, former president Hu Jintao - not related - made a surprise visit to Hu Yaobang's formal residence to pay a tribute. President Xi Jinping's father, Xi Zhongxun, was known as a key ally of Hu Yaobang.

Hu Dehua's comments carry weight because of the respect people still hold for his father - one of the most important leaders responsible for steering China out of the chaos of the Cultural Revolution and into the era of "reform and opening up".

Hu Yaobang died on April 15, 1989, from a heart attack, two years after being elbowed out of power by party conservatives for tolerating "bourgeois liberation".

Hundreds of thousands turned out on the streets to pay their respects. The spontaneous public mourning later transformed into a mass movement for democracy and freedom in Tiananmen Square, but was ended by a brutal military crackdown on June 4.

Hu Dehua said his father did not regret his loss of status, but was heartbroken when reforms that would have brought checks and balances on the power of the government were abandoned.

Hu said every country had at least one key national figure who had led the country towards great progress, and one person in Chinese history who has filled that role was either Sun Yat-sen, who led a revolution to topple the Qing dynasty, or Mao Zedong.

"It remains to be seen who that second person would be," he said. "Hu Yaobang could have been that person, but he was purged."

Twenty-five years after his death, Hu is still best remembered by many for his liberal boldness in freeing China from the strictures of Maoist dogma.

Hu Dehua said one of the most memorable exchanges he had with his father came at the height of the Cultural Revolution in 1968.

The senior Hu asked his teenage son whether he thought the popular slogan of the era - "Everything we do is for Chairman Mao; All our thoughts are of Chairman Mao; and in all our actions we closely follow and obey Chairman Mao" - was correct.

Having seen it published in state newspapers, the younger Hu said he did not question its veracity. "Can't you use your brains? This is clearly problematic," Hu quoted his father as saying. "Everything we do should be for the people, not for Mao."

Forty-six years later, Hu Dehua can still vividly recall how shocked he was when his father uttered these words. "It felt as if I'd been struck by lightning - people dared not speak like that in those days," Hu said of the decade that ensued, when criticism of Mao could result in persecution, prison or death.

"From that moment on, I knew my father was an exceptional man," Hu, 65, said. "He did not follow the herd."

It was perhaps that boldness that earned Hu Yaobang - the Communist Party's top leader from 1980 to 1987 - widespread respect among his countrymen, but also led to his downfall.

Apart from a low-key official ceremony commemorating the 90th anniversary of his birth in 2005, Hu Yaobang's name has scarcely been mentioned in the state press in the past 25 years.

"In the party's official history, it is as though he didn't exist," said Hu, adding that the party "lacks the magnanimity" to reverse its unjust verdict. Asked why he thought the party was reluctant to take the step, he said: "I guess they're afraid of equality, differing opinions and people's rights."

Hu Yaobang had a liberal and pragmatic approach to politics and economics.

As deputy head of the Central Party School and later the party's propaganda chief, he encouraged independent thinking, freeing China from ideological dogmatism and the worship of Mao when the country was still reeling from the Cultural Revolution.

Among the many legacies of his father, Hu Dehua is most proud of his advocacy of democracy and the rule of law, his opposition to repressive rule, his campaign to rehabilitate political victims and the abolition of discrimination against "black five categories" of so-called political enemies.

Hu Yaobang ended the requirement to declare one's class affiliation on official documents.

"People enjoyed freedom from fear for the first time," said Hu Dehua. "I reckon this contribution was no less than [Abraham] Lincoln's liberation of black slaves - he turned the politically condemned class into citizens."