WASHINGTON - Donald Trump has warned China that carrying out a Tiananmen Square-style crackdown on Hong Kong pro-democracy protests would harm trade talks between the two countries, in comments that followed a huge, defiant march through the streets of the global finance hub.

Hong Kong has been rocked by more than two months of protests and clashes between police and hardcore activists who fear the semi-autonomous city's freedoms are under threat from Beijing.

On Sunday, rally organizers said some 1.7 million people turned out for a peaceful march under a sea of umbrellas, defying police orders and stark warnings from China to end the protests.

The American president cautioned that any violent effort by China to quell the unprecedented protests would damage trade negotiations.

"I think it'd be very hard to deal if they do violence, I mean, if it's another Tiananmen Square," Trump told reporters in New Jersey on Sunday. "I think it's a very hard thing to do if there's violence."

Trump's comments came as Washington and Beijing look to revive pivotal high-level talks aimed at ending their trade war.

The trade dispute between the US and China has been blamed for setting world financial markets on edge amid signs of a possible global economic slowdown.

Phone calls between the deputies from two sides are planned for the next 10 days and, if those are successful, negotiations between more senior officials could resume, Trump's chief economic advisor Larry Kudlow said on Sunday.

The demonstrations in Hong Kong have tipped into violence, with protesters paralyzing the city's airport last week and tarnishing a campaign that took pride in its peaceful intent.

Communist Party-ruled mainland China has in turn sharpened its tone towards the dissidents, decrying the "terrorist-like" actions of a violent minority.

State media has broadcast images of military personnel and armored personnel carriers in Shenzhen, across the border from Hong Kong, propaganda read across Hong Kong as intimidation.

China deployed tanks to end student-led protests in the bloody 1989 crackdown in Beijing's Tiananmen Square, resulting in an estimated death toll between several hundred to over a thousand.

If such a situation was repeated in Hong Kong, "I think there'd be... tremendous political sentiment not to do something," Trump said, referring to the trade negotiations with China. -AFP