A former leader of the Tiananmen Square protests in China has lashed out at US presidential hopeful Donald Trump, whom he dubbed a "privileged comeback king", and warned America is at risk of abandoning its cherished freedoms.

Wu'er Kaixi said he was speaking on behalf of Chinese democracy dissidents everywhere in condemning the provocative property magnate, who was criticised last week after describing the 1989 demonstration in Beijing as a "riot".

"Speaking personally, after 27 years in exile from that 'riot'... I think I can speak for all fellow exiled and imprisoned Chinese in condemning Trump," Wu'er wrote in a commentary posted on his official Facebook page.

"I am not alone in appealing to the very same Americans who offered Chinese such as myself refuge when our own government deserted us to put aside partisan disputes and unite against Trump," the activist said.

In a televised exchange, Republican frontrunner Trump was quizzed about his 1990 comments on the student-led protests and subsequent government crackdown that costs hundreds, possibly thousands, of lives, in which he referred to "the power of strength".

Speaking on CNN, Trump insisted he was not endorsing the Chinese Communist Party's brutal response, which saw the military brought in to crush the protests.

"I said that is a strong, powerful government that put it down with strength," he said.

"And then they kept down the riot. It was a horrible thing. It doesn't mean at all I was endorsing it."

Wu'er lived in the US after fleeing China in the aftermath of the protests, but has resided in Taiwan for the past two decades.

He has tried to return to China on several occasions, but has been denied entry each time.

The Communist Party today under President Xi Jinping continues to block any activists advocating democratic reforms, and has tightened its grip on the media.

"Politically, in China, nothing has changed - if anything it has become worse in recent years," Wu'er said.

"Trump, a privileged comeback king from a litany of failed fast-buck business scams, is an enemy of the values that America deeply defines itself by: the same values that have long provided hope to the victims of oppressive power worldwide.

"Those of us who have fought for freedom anywhere in the world worry that something is about to change in America."

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