Hong Hong protesters have appealed to Donald Trump to “liberate” them from China while waving US flags and singing the American national anthem.

In the 14th week of protests, demonstrators marched on the US consulate with a plea for America to pass a bill which would effectively limit trade with China if Hong Long lost its autonomy.

They called on Washington to pass the Hong Kong Democratic and Human Rights Act, to support their cause.

The bill proposes sanctions against Hong Kong and Chinese officials found to suppress democracy and human rights in the city, and could also affect Hong Kong’s preferential trade status with the US.

However to date the president has not been inclined to get involved, despite his ongoing animosity with China over the two countries’ trade negotiations.

Clad in black shirts and wearing masks, some carried posters which read: “President Trump Please Save Hong Kong.”

“We share the same US values of liberty and democracy,” 30-year-old banker David Wong told CNN.

“USA is a country of democracy. Donald Trump is elected by his people. We want this.”

Violence later broke out in the business and retail district as police fired tear gas after protesters vandalised subway stations, set fires and blocked traffic.

Hong Kong has been rocked by three months of unrest sparked by a proposed law that would have allowed criminal suspects to be sent to mainland China for trial.

Many saw the extradition bill as a glaring example of the erosion of civil liberties and rights promised under a “one country, two systems” framework when the former British colony returned to Chinese rule in 1997.

Hong Kong’s government promised last week to formally withdraw the bill, but that failed to appease the demonstrators, who have widened their demands to include calls for direct elections for the city’s leaders and an independent probe into alleged police brutality against protesters.

The unrest has become the biggest challenge to Beijing’s rule since Hong Kong’s return from Britain.

Beijing and the entirely state-controlled media have portrayed the protests as an effort by criminals to split the territory from China, backed by hostile foreigners.

Just before Sunday’s rally ended, violence erupted after riot police detained several people and cleared a crowd from the nearby Central subway station.

Angry protesters smashed glass windows, sprayed graffiti and started a fire at one of the station’s exits.

The government said protesters also set street fires and blocked traffic at some thoroughfares.

In the type of cat-and-mouse battle that has characterised the summer-long protests, riot police pursued groups of protesters down streets, but they kept regrouping.

Police fired multiple rounds of tear gas in the Causeway Bay shopping area after protesters heckled them and refused to leave.

They also searched dozens of young people on the street and inside subway stations.

-with AAP